Some Macs encounter an internal error when compiling cluster_render.glsl
caused by a likely bug in the MVK/Metal
compiler when using gl_HelperInvocation.
-Used a more consistent set of keywords for the shader
-Remove all harcoded entry points
-Re-wrote the GLSL shader parser, new system is more flexible. Allows any entry point organization.
-Entry point for sky shaders is now sky().
-Entry point for particle shaders is now process().
