GLSL: Support Fragment Shader Barycentrics

The two extensions `GLSL_AMD_shader_explicit_vertex_parameter` and `GLSL_NV_fragment_shader_barycentric` both exist in OpenGL.

All features can be implemented in terms of the other one, so it would not matter which extension the SPIR-V shader was written in, the extension ladded would produce an OpenGL shader that can run both on AMD (gcn+) and Nvidia (Turing+).

Start by tracing how SPIR-V Cross handles GLSL extensions and emits OpenGL shaders, then compare the features of GLSL_AMD_shader_explicit_vertex_parameter and GLSL_NV_fragment_shader_barycentric. Done means both extensions are supported and the generated shader can run on the stated AMD and Nvidia targets; the issue names no files or tests.

Written by the indexing model from the issue text.