Apply shadow to roundedRects in RenderEngine.

This CL results in more accurate shadows compared to previous
code, but still does not handle the situation where the shadow
layer has a parent with a round rect clip AND the intersection
of that clip with this layer does not result in a round rect.

Test: atest librenderengine_test
Bug: 186773838
Change-Id: I91dffd82bb4d5837cb20926ff0880dfc183e560c