SoftGPU: Compare stencil correctly based on bitdepth

All sorts of funny rules.  See comments here:
https://github.com/hrydgard/ppsspp/pull/4150

Prescaling the value on read doesn't work, because INCR works with the original value.

Still doesn't completely correct stencil, since so far I don't know if there's anything on the alpha side that actually writes to the alpha channel, heh.

-[Unknown]