[glsl-in] Add gl_VertexIndex (#189)

* [glsl-in] Add gl_VertexIndex

Refactor variable handling out of parser.rs

* [glsl-in] Fixes for gl_VertexIndex

- Make lookup_variable a method of Program

- Try to cleanup conditional logic

* [glsl-in] Error on gl_ builtin in wrong stage

Wrapped in glsl-validate feature