intel: Add acceleration for glDrawPixels(GL_STENCIL_INDEX).

intel: Add acceleration for glDrawPixels(GL_STENCIL_INDEX).

This is nasty because there's no way in GL to output data to the stencil
buffer directly, so we have to do a dance to wrap the depth/stencil buffer
in an ARGB renderbuffer.

Improves performance of several oglconform testcases by better than a factor
of 2.