Fix a ridiculous number of issues and add a test for tev combiner compare mode.

- Now using our own code for synchronizing EFB copies instead of GX (because GX_DrawDone() flushes cached state)
- Removed the pointless call to GX_PixModeSync in CGX_DoEfbCopyTex
- Fix the default alpha combiner register to not enable clamping (like the color combiner)
- Fix GetTevOutput to extract alpha components properly

GetTevOutput alpha reading remains somewhat unreliable due to an off-by-one error due to the limited number of bits in the RGBA6 EFB.