Bug 1534250. Always paint the entire mask. r=mattwoodrow

Previously we would only paint the building area but we
would not do anything to check that the building area had
changed. Since, we're already allocating a surface for the entire
item it's better to just paint it and not worry about doing extra
invalidations.

Originally, we used mVisibleRect here. That was changed to GetPaintRect()
in part 1 of 1460491 and then to GetBuildingRect in part 2. However,
I think the original code was always wrong and we should've been using
mBounds the whole time.

Differential Revision: https://phabricator.services.mozilla.com/D23215

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

layout/painting/nsDisplayList.cpp

@@ -8972,7 +8972,7 @@ bool nsDisplayMasksAndClipPaths::PaintMask(nsDisplayListBuilder* aBuilder,
                                  : imgIContainer::FLAG_SYNC_DECODE_IF_FAST);
   nsRect borderArea = nsRect(ToReferenceFrame(), mFrame->GetSize());
   nsSVGIntegrationUtils::PaintFramesParams params(
-      *aMaskContext, mFrame, GetBuildingRect(), borderArea, aBuilder, nullptr,
+      *aMaskContext, mFrame, mBounds, borderArea, aBuilder, nullptr,
       mHandleOpacity, imgParams);
   ComputeMaskGeometry(params);
   bool painted = nsSVGIntegrationUtils::PaintMask(params);