There are a number of problems with the current OGL function loading code. We
dynamically load all necessary function symbols from libGL.so at startup, but
the code that performs this operation is rather verbose (it was autogenerated
originally, but since then it has been updated manually, which is error prone).
We can use macros to help simplify the function declarations and
initializations. This code currently sits in the GLXFuncs.c file, but we could
easily reorganize this code to sit in a shared OGLFuncs.c file, so that it
will be easier to add the WGL pipeline.

Also, there are a number of (related) minor porting issues that should be
cleaned up as well with this fix, so that the current shared OGL native code
builds properly on Windows. Mostly this involves adding a few explicit casts
in some places so that the Windows compiler doesn't complain.