Avoid unnecessary computation for shadows and clipping for rounded layers

Shadow layers did not need to generate the shader as it was not used
so avoid that overhead.

Setting a roundrect clip and then drawing a single rect of the same size
is always less efficient than just drawing the roundrect directly.

Test: librenderengine_test
Bug: 181028875
Change-Id: I4e333f34b886f630773e4d660973bf8aa4585635