st/xlib: reduce the proliferation of GLX context types

Now there is just a single, struct __GLXcontextRec, which is the
GLXContext typedef has already been defined as a pointer to.  I
believe this is the intended usage, that GLX implementations should
define that struct as they require.

Merge the two previous structs into one and get rid of the
no-longer-necessary type casts and sub-classing.
1 file changed