Enable glsl integer code

Enable glsl integer code

This cl enables true integer support in glsl shaders.
It still uses floating point registers to store all
registers, regardless of the type, so integer and
unsigned integer variables are simply reinterpreted
as integers or unsigned integers and used as such by
the appropriate instructions.

Change-Id: If62213c917b4b0c907e58db9cd36944dd198beaa
Reviewed-on: https://swiftshader-review.googlesource.com/3910
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>