Bug 1587210. Always clear mInvalidRect. r=mstange

In the future mInvalidRect and some other state should move out of DIGroup
and into a non-persistant struct that's only used during Group building.
This will allow us to completely avoid errors like this.

Differential Revision: https://phabricator.services.mozilla.com/D48706

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

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -731,7 +731,6 @@ struct DIGroup {
       }
     }
     mFonts = std::move(fonts);
-    mInvalidRect.SetEmpty();
     aResources.SetBlobImageVisibleArea(
         mKey.value().second(),
         ViewAs<ImagePixel>(mVisibleRect, PixelCastJustification::LayerIsImage));
@@ -1217,6 +1216,8 @@ void Grouper::ConstructGroups(nsDisplayListBuilder* aDisplayListBuilder,
           aCommandBuilder->CreateOrRecycleWebRenderUserData<WebRenderGroupData>(
               item, aBuilder.GetRenderRoot());
 
+      groupData->mFollowingGroup.mInvalidRect.SetEmpty();
+
       // Initialize groupData->mFollowingGroup with data from currentGroup.
       // We want to copy out this information before calling EndGroup because
       // EndGroup will set mLastVisibleRect depending on whether
@@ -1481,6 +1482,8 @@ void WebRenderCommandBuilder::DoGroupingForDisplayList(
   GP("Inherrited scale %f %f\n", scale.width, scale.height);
   GP("Bounds: %d %d %d %d vs %d %d %d %d\n", p.x, p.y, p.width, p.height, q.x,
      q.y, q.width, q.height);
+
+  group.mInvalidRect.SetEmpty();
   if (group.mAppUnitsPerDevPixel != appUnitsPerDevPixel ||
       group.mScale != scale || group.mResidualOffset != residualOffset) {
     GP("Property change. Deleting blob\n");