This adds proper handling of the new
interpolation qualifiers "flat" and
"smooth" for glsl in OpenGL ES 3.0.
These aren't implemented yet, they are
simply parsed properly with this cl.

Change-Id: I32aa2ca0f9ddb4c2b1a1e9c5f285a41a1716f4cc
Reviewed-on: https://swiftshader-review.googlesource.com/2861Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>