nv2a: Implement SET_SHADE_MODEL

[abaire]

Fixes #692
Fixes #912

Unfortunately this involves some non-trivial & ugly changes due to the fact that GL doesn't allow type qualifiers inside of structs, coupled with the fact that on the nv2a the shade model is only applied to a subset of the attributes.

Test
HW results

[abaire]

Draft due to dependence on fix for #912 (the xemu results depart from HW for all of the complex primitives due to the geometry shader expansion). Might be worth merging and doing the geometry shader fix as a followup since it'll add some complexity (I guess it'll need to duplicate the flat shaded attributes across all generated provoking vertices to simulate a single primitive)

UPDATE: I'm wrong, even the basic triangles are using the wrong values, so there's something interesting about what the hardware considers the provoking vertex for flat shading (looks like it uses the first vertex in the primitive instead of the last).

[abaire]

Adjusted the geometry shaders such that the provoking vertex matches hardware. Apart from the usual (practically invisible) differences in blending between my GTX1070 and Xbox 1.0, the output with this PR matches the hardware results, so this is ready for review. #912 is also fixed in this PR due to the change I made to the fixed orientation of the triangles when generating quads (they now match hardware).

[mborgerson]

Thanks! Rebased and fixed merge conflicts