DO NOT MERGE Correctly implement the CLEAR xfermode.

This bug is a regression from the software pipeline and prevents
applications from implementing an "eraser" type tool (for instance
a drawing/painting apps.) This issue affects external applications
when they turn on hardware acceleration.

The previous implementation was using glBlendFunc with the parameters
GL_ZERO/GL_ZERO which doesn't work for text, paths and other alpha
sources (anti-aliasing.) The correct implementation is GL_ZERO/
GL_ONE_MINUS_SRC_ALPHA.

Change-Id: I1446e83480e46174b880120069d76fcad14ba300