glx: Use wrapper macro to detect direct rendering
glx: Use wrapper macro to detect direct rendering

The wrapper macro GC_IS_DIRECT is used in CreateContext and a couple
other places to eliminate the need for some of the '#ifdef
GLX_DIRECT_RENDERING' madness.  There appear to be a *LOT* of places
in glxcmds.c where '#ifdef GLX_DIRECT_RENDERING' is missing.

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