nv50: fix alphatest for non-blendable formats

nv50: fix alphatest for non-blendable formats

The hardware can only do alphatest when using a blendable format. This
means that the various *16 norm formats didn't work with alphatest. It
appears that Talos Principle uses such formats, as well as alpha tests,
for some internal renders, which made them be incorrect. However this
does not appear to affect the final renders, but in a different game it
easily could.

The approach we take is that when alphatests are enabled and a suitable
format is used (which we anticipate is the vast minority of the time),
we insert code into the shader to perform the comparison and discard.
Once inserted, that code lives in the shader forever, and we re-upload
it each time the function changes with a fixed-up compare. To avoid
re-uploading too often, if we switch back to a blendable format, the
test is (effectively) disabled and the hw alphatest functionality is
used.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>