remove SkVM uniform8/16

Uniforms in practice are always pointers or 32-bit ints or floats, so
these are essentially dead code. The change to SkVMBlitter.cpp is the
only interesting change, and I think it makes more sense now than
before.  The program will need float coverage in the end, so might as
well feed it one directly.

Change-Id: I7f1e77731cf10ccc35595012a6df4f9e54a0dad8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/338631
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
5 files changed