Optimize RenderLayer::intersectsDamageRect() slightly

Optimize RenderLayer::intersectsDamageRect() slightly
https://bugs.webkit.org/show_bug.cgi?id=143186

Reviewed by Zalan Bujtas.

We can early return from RenderLayer::intersectsDamageRect() if the
damageRect is empty, since nothing will intersect with the empty rect.

Slight performance gain when scrolling overflow-scroll with lots of nested,
clipping layers.

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::calculateClipRects):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182116 268f45cc-cd09-0410-ab3c-d52691b4dbfc