wgl: Add a test for MakeCurrent returning different function pointers.

Huh, my old rule of "untested code is broken code" didn't hold up, for once.
Prefer non-aliased functions when resolving.

This increases the size of the library, but avoids some of the
concerns that have been brought up with the library silently switching
you from glWhateverEXT() to glWhatever() if there might be slight
behavior differences between the two that hadn't been identified.  The
downside is that it means we can't share the resolver functions among aliases.
Use #defines to shrink the size of gl_generated_dispatch.c

I was running into file size limits with win32 debug symbols.
win32: Delay using dispatch tables until the second MakeCurrent.

Fixes #11.
Disable the dlwrap-based tests on apple.

We'd need a dlvsym() equivalent.