Fix behavior of stencil clipping within clearLayerRegions

bug:16376960

The draw within clearLayerRegions should never be affected by the
current stencil clip, since it's just ensuring that the content of the
layer is cleared, and not doing real content drawing.

Also, add optional verbose GL event logging

Change-Id: I538b1bc631fc091340b76e12db6af0c219851b57