Bug 1704431 - Handle start compositing only when dirty rect exists r=lsalzman,gfx-reviewers

When render_impl is called from update, the device_size is None, and so dirty rect calculation is skipped. Then if RenderCompositorLayersSWGL does not handle start compositing when dirty rect does not exist, we could ignore StartCompositing() from update.

Differential Revision: https://phabricator.services.mozilla.com/D111583

gfx/webrender_bindings/RenderCompositorLayersSWGL.cpp

@@ -75,21 +75,23 @@ void RenderCompositorLayersSWGL::CancelFrame() {
 void RenderCompositorLayersSWGL::StartCompositing(
     const wr::DeviceIntRect* aDirtyRects, size_t aNumDirtyRects,
     const wr::DeviceIntRect* aOpaqueRects, size_t aNumOpaqueRects) {
-  if (!mInFrame) {
+  MOZ_RELEASE_ASSERT(!mCompositingStarted);
+
+  if (!mInFrame || aNumDirtyRects == 0) {
     return;
   }
+
   gfx::IntRect bounds(gfx::IntPoint(0, 0), GetBufferSize().ToUnknownSize());
   nsIntRegion dirty;
-  if (aNumDirtyRects) {
-    for (size_t i = 0; i < aNumDirtyRects; i++) {
-      const auto& rect = aDirtyRects[i];
-      dirty.OrWith(gfx::IntRect(rect.origin.x, rect.origin.y, rect.size.width,
-                                rect.size.height));
-    }
-    dirty.AndWith(bounds);
-  } else {
-    dirty = bounds;
+
+  MOZ_RELEASE_ASSERT(aNumDirtyRects > 0);
+  for (size_t i = 0; i < aNumDirtyRects; i++) {
+    const auto& rect = aDirtyRects[i];
+    dirty.OrWith(gfx::IntRect(rect.origin.x, rect.origin.y, rect.size.width,
+                              rect.size.height));
   }
+  dirty.AndWith(bounds);
+
   nsIntRegion opaque(bounds);
   opaque.SubOut(mWidget->GetTransparentRegion().ToUnknownRegion());
   for (size_t i = 0; i < aNumOpaqueRects; i++) {