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/image/tests.rs

@@ -2,12 +2,18 @@
 // Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>
 
 use super::*;
+use crate::render::Canvas;
 
 fn solid_rgba( w: u32, h: u32 ) -> Arc<Vec<u8>>
 {
 	Arc::new( vec![ 255u8; ( w * h * 4 ) as usize ] )
 }
 
+fn dummy_canvas() -> Canvas
+{
+	Canvas::new( 800, 600 )
+}
+
 // ── builders / defaults ───────────────────────────────────────────────────
 
 #[ test ]
@@ -57,8 +63,7 @@ fn preferred_size_default_scales_height_to_match_width_aspect()
 {
 	// 200×100 source image at max_width = 400 → scale = 2 → height 200.
 	let img = Image::new( solid_rgba( 200, 100 ), 200, 100 );
-	let canvas = Canvas::new( 480, 900 );
-	let ( w, h ) = img.preferred_size( 400.0, &canvas );
+	let ( w, h ) = img.preferred_size( 400.0, &dummy_canvas() );
 	assert_eq!( w, 400.0 );
 	assert_eq!( h, 200.0 );
 }
@@ -67,8 +72,7 @@ fn preferred_size_default_scales_height_to_match_width_aspect()
 fn preferred_size_with_explicit_size_wins_over_aspect_logic()
 {
 	let img = Image::new( solid_rgba( 200, 100 ), 200, 100 ).size( 50.0, 25.0 );
-	let canvas = Canvas::new( 480, 900 );
-	assert_eq!( img.preferred_size( 1000.0, &canvas ), ( 50.0, 25.0 ) );
+	assert_eq!( img.preferred_size( 1000.0, &dummy_canvas() ), ( 50.0, 25.0 ) );
 }
 
 #[ test ]
@@ -84,8 +88,7 @@ fn preferred_size_cover_mode_keeps_aspect_ratio()
 {
 	// 100×50 source in cover mode at max_width = 300 → height = 300 * (50/100) = 150.
 	let img = Image::new( solid_rgba( 100, 50 ), 100, 50 ).cover();
-	let canvas = Canvas::new( 480, 900 );
-	let ( w, h ) = img.preferred_size( 300.0, &canvas );
+	let ( w, h ) = img.preferred_size( 300.0, &dummy_canvas() );
 	assert_eq!( w, 300.0 );
 	assert_eq!( h, 150.0 );
 }
@@ -98,8 +101,8 @@ fn preferred_size_cover_and_default_match_for_uniform_scaling()
 	// when the source is taller than wide.
 	let normal  = Image::new( solid_rgba( 100, 50 ), 100, 50 );
 	let covered = Image::new( solid_rgba( 100, 50 ), 100, 50 ).cover();
-	let canvas = Canvas::new( 480, 900 );
-	assert_eq!( normal.preferred_size( 200.0, &canvas ), covered.preferred_size( 200.0, &canvas ) );
+	let c = dummy_canvas();
+	assert_eq!( normal.preferred_size( 200.0, &c ), covered.preferred_size( 200.0, &c ) );
 }
 
 // ── Arc sharing ───────────────────────────────────────────────────────────