Embedder primitives: placed-child clipping, offscreen RGBA readback, standalone text measurement

Three additions an embedder needs to drive ltk as the render backend for a retained, externally-owned widget tree, each kept general rather than tied to one consumer.
Stack placed-child clipping. `Stack::push_placed_clipped(e, rect, clip)` places a child at an exact rect like `push_placed` but clips its subtree's drawing to `clip` (in the Stack's coordinate space) — Android's clipChildren: content that overflows, such as a scrolled list row reaching above the list or an inner card past a rounded bubble, is not painted. The Stack child tuple grows an 8th `Option<Rect>` for the clip, and `layout_and_draw` brackets a clipped child with `set_clip_rects`/restore around the recursion. The clip is shifted by the Stack's own origin to match the placed rect (which `Stack::layout` already offsets), so a Stack laid out at a non-zero origin clips in the right place rather than off by that origin.
Offscreen RGBA readback. `Canvas::read_rgba_pixels(out)` reads any canvas into tightly packed straight-alpha RGBA8, top-left row first. Unlike `read_gles_rgba_pixels` it also serves the software backend, un-premultiplying its pixmap, so an offscreen software canvas (an embedder's scratch bitmap) can be read back into a straight-alpha buffer. `Canvas::is_software()` lets a caller branch on the backend — e.g. to honour a real path clip on software but only a bounding rect on GLES.
Standalone text measurement. `measure_text(text, size)`, re-exported at the crate root, measures one line with the default UI font and the system fallback chain, returning `(width, line_height)` in pixels without a live `Canvas` — for an embedder's measure pass that must produce the same metrics the renderer will later use. It is backed by `system_fonts::primary_handle()`, a process-wide cached handle for the primary UI font (the same default a canvas loads, with the bundled-font fallback), which widens `render::helpers::load_default_font_bytes` to `pub(crate)`.

src/layout/stack.rs

@@ -52,10 +52,11 @@ pub enum VAlign
 pub struct Stack<Msg: Clone>
 {
 	/// Children with their alignment, margin, and extra `(x, y)` translation
-	/// applied after alignment. Drawn in order — last child is on top. The final
+	/// applied after alignment. Drawn in order — last child is on top. The 7th
 	/// `Option<Rect>` overrides alignment/sizing with an exact rect (see
-	/// [`push_placed`](Self::push_placed)).
-	pub children: Vec<( Element<Msg>, HAlign, VAlign, f32, f32, f32, Option<Rect> )>,
+	/// [`push_placed`](Self::push_placed)); the 8th clips the child's draw to a
+	/// rect (Android clipChildren — see [`push_placed_clipped`](Self::push_placed_clipped)).
+	pub children: Vec<( Element<Msg>, HAlign, VAlign, f32, f32, f32, Option<Rect>, Option<Rect> )>,
 	/// When `true`, [`preferred_size`](Self::preferred_size) reports the max of
 	/// children's intrinsic widths and heights instead of `(max_width, tallest)`.
 	pub fit_content: bool,
@@ -102,7 +103,7 @@ impl<Msg: Clone> Stack<Msg>
 		margin:  f32,
 	) -> Self
 	{
-		self.children.push( ( e.into(), h_align, v_align, margin, 0.0, 0.0, None ) );
+		self.children.push( ( e.into(), h_align, v_align, margin, 0.0, 0.0, None, None ) );
 		self
 	}
 
@@ -119,7 +120,7 @@ impl<Msg: Clone> Stack<Msg>
 		offset_y: f32,
 	) -> Self
 	{
-		self.children.push( ( e.into(), h_align, v_align, 0.0, offset_x, offset_y, None ) );
+		self.children.push( ( e.into(), h_align, v_align, 0.0, offset_x, offset_y, None, None ) );
 		self
 	}
 
@@ -133,7 +134,22 @@ impl<Msg: Clone> Stack<Msg>
 		rect: Rect,
 	) -> Self
 	{
-		self.children.push( ( e.into(), HAlign::Start, VAlign::Top, 0.0, 0.0, 0.0, Some( rect ) ) );
+		self.children.push( ( e.into(), HAlign::Start, VAlign::Top, 0.0, 0.0, 0.0, Some( rect ), None ) );
+		self
+	}
+
+	/// Like [`push_placed`](Self::push_placed) but clips the child's drawing to
+	/// `clip` (in the Stack's coordinate space). Mirrors Android's clipChildren:
+	/// content that overflows the clip — e.g. a scrolled list row reaching above
+	/// the list, or an inner card past a rounded bubble — is not painted.
+	pub fn push_placed_clipped(
+		mut self,
+		e:    impl Into<Element<Msg>>,
+		rect: Rect,
+		clip: Rect,
+	) -> Self
+	{
+		self.children.push( ( e.into(), HAlign::Start, VAlign::Top, 0.0, 0.0, 0.0, Some( rect ), Some( clip ) ) );
 		self
 	}
 
@@ -142,7 +158,7 @@ impl<Msg: Clone> Stack<Msg>
 		if self.fit_content
 		{
 			let content_w = self.children.iter()
-				.map( |( c, _, _, _, _, _, _ )| match c
+				.map( |( c, .. )| match c
 				{
 					Element::Spacer( s )      => s.resolved_width( canvas ).unwrap_or( 0.0 ),
 					Element::Separator( _ )   => 0.0,
@@ -157,13 +173,13 @@ impl<Msg: Clone> Stack<Msg>
 				} )
 				.fold( 0.0_f32, f32::max );
 			let max_h = self.children.iter()
-				.map( |( c, _, _, _, _, _, _ )| c.preferred_size( max_width, canvas ).1 )
+				.map( |( c, .. )| c.preferred_size( max_width, canvas ).1 )
 				.fold( 0.0_f32, f32::max );
 			return ( content_w.min( max_width ), max_h );
 		}
 
 		let max_h = self.children.iter()
-			.map( |( c, _, _, _, _, _, _ )| c.preferred_size( max_width, canvas ).1 )
+			.map( |( c, .. )| c.preferred_size( max_width, canvas ).1 )
 			.fold( 0.0_f32, f32::max );
 		( max_width, max_h )
 	}
@@ -171,7 +187,7 @@ impl<Msg: Clone> Stack<Msg>
 	/// Return `(rect, child_index)` pairs, computing each child's rect from its alignment.
 	pub fn layout( &self, rect: Rect, canvas: &Canvas ) -> Vec<(Rect, usize)>
 	{
-		self.children.iter().enumerate().map( |( i, ( child, h_align, v_align, margin, ox, oy, placed ) )|
+		self.children.iter().enumerate().map( |( i, ( child, h_align, v_align, margin, ox, oy, placed, .. ) )|
 		{
 			if let Some( p ) = placed
 			{
@@ -213,8 +229,8 @@ impl<Msg: Clone> Stack<Msg>
 		Stack
 		{
 			children: self.children.into_iter()
-				.map( |( child, ha, va, margin, ox, oy, placed )|
-					( child.map_arc( f ), ha, va, margin, ox, oy, placed ) )
+				.map( |( child, ha, va, margin, ox, oy, placed, clip )|
+					( child.map_arc( f ), ha, va, margin, ox, oy, placed, clip ) )
 				.collect(),
 			fit_content: self.fit_content,
 		}