Make the GLSL compiler independent of the API shader class.

Bug 18962347

Change-Id: I1a45c5572a45a16ae04ab77e7029ab31905ac702
Reviewed-on: https://swiftshader-review.googlesource.com/1750
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>