mesa: Implement GL_KHR_blend_equation_advanced_coherent.

This adds the extension enable (so drivers can advertise it) and the
extra boolean state flag, GL_BLEND_ADVANCED_COHERENT_KHR, which can
be set to request coherent blending.

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