gallium: prepare for per-rendertarget blend enables, writemasks, blend funcs

gallium: prepare for per-rendertarget blend enables, writemasks, blend funcs

GL 3.0 (EXT_draw_buffers2) and other APIs allow independent blend enables and
write masks per render target, ARB_draw_buffers_blend (and other APIs) also
allow independent blend functions. Things like dithering, logic ops however
are not extended to be per rendertarget, that might be conceptually possible
however it doesn't look like any API wants to expose this.