Fallback to driver glX functions when libGL wrappers don't exist

If a driver supplies a glX function, but the libGL wrapper code
doesn't exist (possibly due to an un-supported architecture), then
just return the driver function as that will at least work for
applications using a single driver.

This wasa found on RISC-V, which doesn't have native libglvnd support
yet, and when applications request glX functions that are provided by
libGL, like glXSwapIntervalEXT. I think the problem is that on RISC-V,
we cannot generate wrappers on the fly for these, and so we end up
with a NULL libGL wrapper function, which causes applications to crash.

Signed-off-by: Keith Packard <keithp@keithp.com>
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