Updates to libGLX vendor library interface

Created by: kbrenneman

This is an assortment of changes to the libGLX interface.

The threadAttach callback is probably the most critical, since it provides a more general workaround to a lot of broken applications.

The others are as much cleanup as anything else.

One question that's been raised is whether to keep a single version number or to split it into a major/minor pair, with the usual pattern of incrementing the minor version for backward-compatible changes.

And if we do add a minor version number, should it get passed to __glx_Main as a separate parameter (changing the function signature)? It would also be possible to just put the major and minor version numbers into the high- and low-order 16 bits.