glsl: add support for ARB_blend_func_extended (v3)

glsl: add support for ARB_blend_func_extended (v3)

This adds index support to the GLSL compiler.

I'm not 100% sure of my approach here, esp without how output ordering
happens wrt location, index pairs, in the "mark" function.

Since current hw doesn't ever have a location > 0 with an index > 0,
we don't have to work out if the output ordering the hw requires is
location, index, location, index or location, location, index, index.
But we have no hw to know, so punt on it for now.

v2: index requires layout - catch and error
    setup explicit index properly.

v3: drop idx_offset stuff, assume index follow location

Signed-off-by: Dave Airlie <airlied@redhat.com>