New, separate GPU plugin based on Unai. (#233)

This new plugin is based on Unai but is more accurate and fixes a few issues.
According to some tests on real hardware :
fps      old new
spyro1   130 112
tekken3   95  68
nfs3     107  91

Because of this, it was decided to make it separate from the "Old" Unai.

Note that this doesn't have the threading changes from libretro's fork yet :
this will be for another PR.

Co-authored-by: negativeExponent <negativeExponent@users.noreply.github.com>
Co-authored-by: Justin Weiss <justin@justinweiss.com>
Co-authored-by: senquack <dansilsby@gmail.com>