Fix rendering issue with layers.

A change was made earlier in the renderer to not snap the current clip rect
on every clip operation. The clip rect must not be snapped to pixel boundaries
before being used for pixel precision operations. Some operations were doing
this correctly, others were not. For instance, a layer was always created by
rounding to the correct pixels, but composing a layer was done with a non
rounded clip, thus resulting in slightly odd behaviors.

Change-Id: I31c2cf0a343450470bcd449c3f13fe37648b9c7c