Bug 1571862 - When the layer change region is too complex, always set mInvalidRegion to mRenderBounds, even if we're currently rendering to mTarget. r=mattwoodrow

We only use the result of this calculation for composites to the actual window
(and stash it for later if this composite is to an external target), so
mTargetBounds is always unrelated.

Differential Revision: https://phabricator.services.mozilla.com/D40871

--HG--
extra : moz-landing-system : lando

gfx/layers/composite/LayerManagerComposite.cpp

@@ -564,7 +564,7 @@ void LayerManagerComposite::UpdateAndRender() {
         mRoot, changed, nullptr);
 
     if (overflowed) {
-      changed = mTarget ? mTargetBounds : mRenderBounds;
+      changed = mRenderBounds;
     }
 
     mInvalidRegion.Or(mInvalidRegion, changed);