Fix subsurfaces not receiving position updates

This fixes an issue where subsurfaces would always stay at their
original location regardless of `wl_subsurface::set_position` calls.

This was caused by the subsurface's texture being loaded from cache with
its original location. To ensure the subsurface position is correct, the
texture's location is now always updated after loading it from the
cache.

Closes #145.

src/drawing.rs

@@ -1,5 +1,6 @@
 //! Drawing utilities.
 
+use std::cell::Cell;
 use std::ops::Deref;
 use std::rc::Rc;
 
@@ -54,7 +55,7 @@ const GESTURE_HANDLE_BLOCKED_RGBA: [u8; 3] = [117, 42, 42];
 pub struct Texture {
     opaque_regions: Vec<Rectangle<i32, Logical>>,
     tracker: DamageSnapshot<i32, Logical>,
-    location: Point<i32, Logical>,
+    location: Cell<Point<i32, Logical>>,
     src_rect: Rectangle<f64, Logical>,
     dst_size: Size<i32, Logical>,
     buffer_size: Size<i32, Logical>,
@@ -118,10 +119,10 @@ impl Texture {
         Self {
             opaque_regions,
             window_scale,
-            location,
             texture,
             tracker: buffer.damage.tracker.snapshot(),
             buffer_size: buffer.buffer_size,
+            location: Cell::new(location),
             buffer: buffer.buffer.clone(),
             transform: buffer.transform,
             scale: buffer.scale as f64,
@@ -146,6 +147,11 @@ impl Texture {
         Texture::new(texture, logical_size, scale, opaque)
     }
 
+    /// Move texture to a different location.
+    pub fn set_location(&self, location: Point<i32, Logical>) {
+        self.location.replace(location);
+    }
+
     /// Target size after rendering.
     pub fn size(&self) -> Size<i32, Logical> {
         self.dst_size
@@ -185,7 +191,8 @@ impl Element for RenderTexture {
 
     fn geometry(&self, scale: Scale<f64>) -> Rectangle<i32, Physical> {
         let scale = self.window_scale.map_or(scale.x, |window_scale| window_scale.scale(scale.x));
-        Rectangle::from_loc_and_size(self.location, self.dst_size).to_physical_precise_round(scale)
+        Rectangle::from_loc_and_size(self.location.get(), self.dst_size)
+            .to_physical_precise_round(scale)
     }
 
     fn damage_since(

src/windows/window.rs

@@ -275,7 +275,11 @@ impl<S: Surface + 'static> Window<S> {
 
                 // Skip surface if buffer was already imported.
                 if let Some(texture) = &data.texture {
+                    // Ensure texture's location is up to date.
+                    texture.set_location(location);
+
                     self.texture_cache.push(texture.clone(), location);
+
                     return TraversalAction::DoChildren(location);
                 }