blend equation advanced
new nir pass and corresponding tex instruction to lower
tex_ms_fb.. the rest is freedreno specific.
Compared to i965 and codegen backend code which does something similar for
load_output, this doesn't handle more than a single MRT, which is not supported in the KHR_blend_equation_advanced extension. Not sure if there is any other gles extension which would expose this? It might be possible to set things up as an array of samplers but sampling from tile buffer has some special magic so I'm not 100% sure if
load_output plus MRT is possible for me. Which might limit the usefulness of this pass for i965/codegen. If it is not useful to others, I don't mind moving it to ir3.