__glDispatchCheckMultithreaded will now check if the thread's current dispatch
table is NULL, and if it is, it will plug in the no-op table.

This is a workaround for some broken applications which try to call OpenGL
functions without a current context. As long as the thread calls a GLX function
first, any OpenGL calls it makes later will call a no-op stub instead of just
crashing.

Added calls to __glXThreadInitialize to some of the functions in
__GLXapiExports, so that GLX extension functions will still call
__glXThreadInitialize.