etnaviv: polygon offset and alpha blending fixes
Fixes many failures in dEQP-GLES2.functional.fragment_ops.blend.*
- Anything involving constant color
- Cases using non-ADD blend function with ONE/ZERO factors, and cases where alpha function differs from rgb function.
Also fixes cases with using polygon offset, such as dEQP-GLES2.functional.polygon_offset.fixed16_displacement_with_units
Edited by Jonathan Marek