Remove dependency on SPIRV-Cross for OpenGL scenarios

[syntheticmagus]

When the target rendering path is OpenGL, SPIRV-Cross should not actually be necessary because glslang should be sufficient to do all our required conversions on its own. Note that to make this change, all of the logic in NativeEngine::CreateProgram that depends on SPIRV-Cross types will need to be rewritten to use glslang instead; that logic should probably also be pulled out of NativeEngine, too. This is therefore expected to be a fairly large change that will affect behavior in every rendering path because it will change how the bgfx shader representations are created.

[bghgary]

Reopen if we have a need to make this work.