perf(compositor): reuse the kept background's texture on Windows and macOS

A cache miss allocated a new full-size texture each time, so a key that
changes every frame (a blur or colour dragged in the inspector) allocated
one per frame. Take the previous one back, as Linux does; resized() empties
the cache, so its size always matches.

Author: EtienneLescot

crates/compositor/src/compositor_macos.rs

@@ -2409,14 +2409,18 @@ impl Compositor {
                 .then(|| crate::frame_geometry::BackgroundKey::of(bg_desc, bg_blur, lp.bg_color, [rw, rh]))
                 .flatten();
             if let Some(key) = key {
-                let tex = make_texture(
-                    &self.gpu.device,
-                    metal::MTLPixelFormat::RGBA8Unorm,
-                    self.render_w,
-                    self.render_h,
-                    metal::MTLStorageMode::Private,
-                    metal::MTLTextureUsage::ShaderRead,
-                );
+                // L'ancienne texture est reprise : `resized` vide le cache, sa taille est la bonne.
+                let old = self.bg_cache.borrow_mut().take().map(|c| c.tex);
+                let tex = old.unwrap_or_else(|| {
+                    make_texture(
+                        &self.gpu.device,
+                        metal::MTLPixelFormat::RGBA8Unorm,
+                        self.render_w,
+                        self.render_h,
+                        metal::MTLStorageMode::Private,
+                        metal::MTLTextureUsage::ShaderRead,
+                    )
+                });
                 self.blit_background(cmd_buf, &tex, false);
                 *self.bg_cache.borrow_mut() = Some(BgCache { key, tex });
             }

crates/compositor/src/compositor_windows.rs

@@ -973,12 +973,19 @@ impl Compositor {
 
     /// Garde le fond que le RT porte à cet instant (`bg_cache`), sous `key`.
     unsafe fn keep_background(&self, key: crate::frame_geometry::BackgroundKey) -> Result<()> {
-        let mut d = D3D11_TEXTURE2D_DESC::default();
-        self.rt.GetDesc(&mut d);
-        d.BindFlags = 0;
-        let mut tex: Option<ID3D11Texture2D> = None;
-        self.dev.CreateTexture2D(&d, None, Some(&mut tex))?;
-        let tex = tex.ok_or_else(|| anyhow::anyhow!("CreateTexture2D (fond gardé)"))?;
+        // L'ancienne texture est reprise : `resized` vide le cache, sa taille est la bonne.
+        let old = self.bg_cache.borrow_mut().take().map(|c| c.tex);
+        let tex = match old {
+            Some(tex) => tex,
+            None => {
+                let mut d = D3D11_TEXTURE2D_DESC::default();
+                self.rt.GetDesc(&mut d);
+                d.BindFlags = 0;
+                let mut tex: Option<ID3D11Texture2D> = None;
+                self.dev.CreateTexture2D(&d, None, Some(&mut tex))?;
+                tex.ok_or_else(|| anyhow::anyhow!("CreateTexture2D (fond gardé)"))?
+            }
+        };
         self.ctx.CopyResource(&tex, &self.rt);
         *self.bg_cache.borrow_mut() = Some(BgCache { key, tex });
         Ok(())