[Enhancement] Use GLSLangValidator to validate OpenGL Shaders

[thargy]

Currently ShaderGen.App only uses glslangvalidator to compile Vulkan. It would be nice to have the option to use it to validate the OpenGL ES and 330 too.

We could do this by extending #93 to include a new option:

V | Validate Validates OpenGLES And OpenGL330 shaders, and in the case of OpenGL450 it only validates and doesn't compile spirv, just outputting the shader code. For other backends is the same as R except does not compile (so requires the output of uncompiled shader).