render: sub-canvases inherit the root layout viewport for fluid resolution

Content drawn inside a scroll rendered smaller than identical content outside it: the scroll viewport draws its child into a sub-canvas sized to the viewport rect, and Canvas::viewport_layout / viewport_logical — the viewports that geom_px and font_px resolve fluid Lengths against — returned the canvas's own size. Inside a 312 px sub-canvas on a 360 px surface every fluid geometry (icon sizes, row heights, paddings) and font size resolved against 312 instead of 360, shrinking the scroll content by the width ratio, ~13 % in a phone-sized window with 24 px margins. First observed in Eydos Settings' Wi-Fi list, where the connected network row sits outside the scroll and the rest inside: the same full-fan signal icon measured 21x18 px outside and 18x16 px inside.
Add a layout_viewport field to SoftwareCanvas and GlesCanvas, None on root canvases and set by sub_canvas to the parent's effective layout viewport, so nested sub-canvases keep propagating the root surface size. viewport_layout and viewport_logical consult the inherited value before falling back to the canvas size. Root canvases are untouched, and the GLES clip layers, which reuse sub_canvas, get the same correction.

src/gles_render/mod.rs

@@ -151,6 +151,12 @@ pub struct GlesCanvas
 	/// falls back to [`Self::font`].
 	pub font_registry: Option<Arc<FontRegistry>>,
 	pub dpi_scale:    f32,
+	/// Layout-resolution viewport inherited from the parent canvas.
+	/// `None` on a root canvas (resolve against own size); sub-canvases
+	/// carry the root surface size so fluid geometry and fonts resolve
+	/// the same inside offscreen content (scroll viewports, clip layers)
+	/// as outside it.
+	pub( crate ) layout_viewport: Option<( f32, f32 )>,
 	pub global_alpha: f32,
 	pub width:        u32,
 	pub height:       u32,