mesa: add support for ARB_blend_func_extended (v4)
Add implementations of the two API functions,
Add a new strings to uint mapping for index bindings
Add the blending mode validation for SRC1 + SRC_ALPHA_SATURATE
Add get for MAX_DUAL_SOURCE_DRAW_BUFFERS
v2:
Add check in valid_to_render to address case in spec ERRORS.
v3:
Add index to ir.h so this patch compiles on its own
fixup comment
v4: fixup Brian's comments
The GLSL patch will setup the indices.
Signed-off-by: Dave Airlie <airlied@redhat.com>