Correctly implement the CLEAR xfermode.

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: I4cca65e57b6a37bbf5a41d382cb0648ee8e11e79