Unused OutputVertex attributes were being left un-initialized. The
leftover garbage sometimes decoded as floating-point denormalized
values, causing fallbacks to microcode and massive slowdowns in the rest
of the rasterization pipeline even though the results were unused. By
zeroing the structure we ensure these attributes only contain harmless
zeros.

More specifically, this also fixes crashes by Citra trying to load a src2 register even if the current instruction does not use that.

This allows for proper emulation of the different CMP/LRP/MAD instructions.