compiler: Add a new STATE_VAR_ADVANCED_BLENDING_MODE built-in uniform.

compiler: Add a new STATE_VAR_ADVANCED_BLENDING_MODE built-in uniform.

This will be used for emulating GL_KHR_advanced_blend_equation features
in shader code.  We'll pass in the blending mode that's in use, and use
that in (effectively) a switch statement in the shader.

v2: Use the new _AdvancedBlendMode field.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>