ltk: subsurface slides over overlays, axis-locked swipes, physical-space layout, touch reset on resume

Subsurfaces can now be parented to an overlay surface, not just the main surface. `SubsurfaceSpec` gains `parent: SubsurfaceParent { Main, Overlay(id) }` so a sliding panel can ride above app windows the way an overlay panel does, and an optional `gpu: bool` so content that uses the `surface-panel` backdrop-filter glass (a GLES-only pass) keeps it while sliding instead of dropping to the software rasteriser. `reconcile_subsurfaces` resolves each spec's parent independently — skipping an `Overlay` parent that is absent, unconfigured, or zero-sized — tracks the rastered size on the slot, and commits each touched parent once per frame. The ~14 GLES shader programs are compiled once into a shared `AppData::subsurface_gles_canvas` reused across every subsurface, so a lazily re-created sliding panel never recompiles them (hundreds of ms on a mobile GPU).
Vertical and horizontal swipes are now mutually exclusive: a gesture locks onto its dominant axis within the first 8 px of travel (new `SwipeAxis`) and ignores the perpendicular axis for the rest of the gesture, so a vertical swipe that drifts sideways no longer also drives the pager (and vice-versa). The upward swipe progress is no longer clamped at 1.0 — follow-the-finger panels can keep tracking the finger past the commit threshold — and a release below threshold delivers a final `progress = 0.0` cancellation pulse. A vertical swipe also no longer re-rasters the full-screen main surface on every motion event: only the overlays it drives are refreshed, while the horizontal pager (which does move the main surface) still redraws it. Re-rastering the main on every frame of a vertical drag was wasted work that stalled the loop and made the gesture feel laggy to start on a slow GPU.
Layout-affecting `Length` values (widths, paddings, gaps, widget sizes, including `Vw` / `Vh`) now resolve against the physical viewport via the new `Canvas::viewport_layout()` — the space the layout tree is actually computed in — so a `Vw(100)` fills the surface on a HiDPI (scale 2) output instead of covering half of it. Font sizes are unchanged: they still resolve against the logical viewport and are scaled at raster time. Switched column, row, spacer, wrap_grid, container, image, and vslider over to it; `img_widget` and `vslider` now document `Length::vw` / `vh` sizing and the showcase example demonstrates a viewport-relative image.
Touch gesture state is reset when the touch capability is added or removed — suspend / resume on devices that power the touchscreen down. A yanked capability never delivers the pending `up` / `cancel`, so the shared `reset_touch_state()` (also used by the `wl_touch.cancel` handler) drops the stranded `primary_touch_id` / slot state across the main surface and every overlay, keeping the first post-resume gesture clean. Also drops an accidental duplicate `on_scale_changed` call from the scale-change handler.

src/widget/vslider/mod.rs

@@ -65,9 +65,9 @@ pub struct VSlider<Msg: Clone>
 	/// Current value in `[0.0, 1.0]`. `0.0` paints no fill; `1.0` fills the
 	/// whole pill.
 	pub value:     f32,
-	/// Fixed width of the pill in logical units. Defaults to 56 px.
+	/// Width of the pill. Defaults to 56 px; accepts any [`Length`].
 	pub width:     Length,
-	/// Fixed height of the pill in logical units. Defaults to 160 px.
+	/// Height of the pill. Defaults to 160 px; accepts any [`Length`].
 	pub height:    Length,
 	/// Callback invoked with the new value when the slider is tapped or
 	/// dragged. `Arc` (not `Box`) so the layout pass can clone it into the
@@ -117,9 +117,10 @@ impl<Msg: Clone> VSlider<Msg>
 		self
 	}
 
-	/// Override the fixed pill `(width, height)` in logical units. Both are
-	/// clamped to a minimum of `2.0` after viewport-relative values resolve,
-	/// so a rounded pill can always be drawn.
+	/// Override the pill size. Accepts logical `f32` pixels or any [`Length`]
+	/// variant (e.g. `Length::vw(16.0)` for 16 % of the viewport width). Both
+	/// are clamped to a minimum of `2.0` after viewport-relative values
+	/// resolve, so a rounded pill can always be drawn.
 	pub fn size( mut self, width: impl Into<Length>, height: impl Into<Length> ) -> Self
 	{
 		self.width  = width.into();
@@ -138,11 +139,11 @@ impl<Msg: Clone> VSlider<Msg>
 	/// the type-level docs on intrinsic sizing.
 	pub fn preferred_size( &self, _max_width: f32, canvas: &Canvas ) -> (f32, f32)
 	{
-		let viewport = canvas.viewport_logical();
-		(
-			self.width.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 2.0 ),
-			self.height.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 2.0 ),
-		)
+		let vp = canvas.viewport_layout();
+		let em = Length::EM_BASE_DEFAULT;
+		let w  = self.width.resolve( vp, em ).max( 2.0 );
+		let h  = self.height.resolve( vp, em ).max( 2.0 );
+		( w, h )
 	}
 
 	/// Compute the value `[0.0, 1.0]` from a tap/drag y position within `rect`.