lima: implement dual-source blend

It was a bit trickier to RE, since blob doesn't expose this functionality at all, however we had a clue from the very beginning: lima_blend_factor is 3 bits, i.e. 8 values, but only 5 of them were used, it just waited till someone tried what 3 unused values do.

Interestingly enough, it turns out "5" works just as "0" (which is PIPE_BLENDFACTOR_SRC_), but only if output register for gl_FragColor is $0, So it looks suspiciously similar with PIPE_BLENDFACTOR_SRC1_ behavior, and looks like secondary output is taken from $0.

Since output regs for all other outputs are configured via RSW, there must be a field in RSW for output register for secondary color, it's likely 4 bits and it's currently set to 0 for reg $0.

Then it was just a matter of brute-forcing various consecutive 4 bits in RSW - and indeed, setting top 4 bits of rsw->aux0 to the index of gl_FragColor output register fixes blending tests when we use "5" blend factor instead of "0".

So it must be a register number for gl_SecondaryFragColor. Unlike gl_FragColor, the field is only repeated once in RSW.