This code requires flat interpolation to work properly, but otherwise
R16I and R16U types should work properly with this code. Other enums
added here (remaining combinations of R/GR/ABGR, 8/16, I/U) aren't
implemented yet.

Change-Id: Ica4e29d0421611e3178a20aadc01f3645dc793bf
Reviewed-on: https://swiftshader-review.googlesource.com/5460Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>