Bug 1948882 - Fix incorrect opaque swapchain layer on empty composites of transparent windows r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D239476

gfx/wr/webrender/src/renderer/mod.rs

@@ -3643,10 +3643,10 @@ impl Renderer {
             }
         }
 
-        // If no tiles were present, add an empty layer to force
+        // If no tiles were present, and we expect an opaque window,
-        // a composite that clears the screen, to match existing
+        // ddd an empty layer to force a composite that clears the screen,
-        // semantics.
+        // to match existing semantics.
-        if input_layers.is_empty() {
+        if window_is_opaque && input_layers.is_empty() {
             input_layers.push(CompositorInputLayer {
                 usage: CompositorSurfaceUsage::Content,
                 is_opaque: true,