Initial version.

Provide exported functions for glXSwapIntervalEXT(), glXSwapIntervalSGI()
and glXSwapIntervalMESA(). Also hook into dlsym(), dlvsym(),
glXGetProcAddress() and glXGetProcAddressARB() so that we cover all
possible paths an application might use to get to these functions.

The debug build also  hooks into glXSwapBuffers for testing purposes.