Bug 1854467 - Avoid using ExtractSubrect with OffscreenCanvas. r=gfx-reviewers,jrmuizel

With OffscreenCanvas, mCanvasElement may be null, so we need to check
for that case when deciding whether or not we need to use ExtractSubrect
when calling DrawImage.

Differential Revision: https://phabricator.services.mozilla.com/D188877

dom/canvas/CanvasRenderingContext2D.cpp

@@ -5461,7 +5461,8 @@ void CanvasRenderingContext2D::DrawImage(const CanvasImageSource& aImage,
 
   if (srcSurf) {
     gfx::Rect sourceRect(aSx, aSy, aSw, aSh);
-    if (element == mCanvasElement) {
+    if ((element && element == mCanvasElement) ||
+        (offscreenCanvas && offscreenCanvas == mOffscreenCanvas)) {
       // srcSurf is a snapshot of mTarget. If we draw to mTarget now, we'll
       // trigger a COW copy of the whole canvas into srcSurf. That's a huge
       // waste if sourceRect doesn't cover the whole canvas.