Set the environment variable __GLX_FORCE_VENDOR_LIBRARY_0 for all of the GLX
tests instead of __GLX_VENDOR_LIBRARY_NAME.
Individual tests can force libGLX to load a vendor library early by calling
glXGetClientString or similar.
Added a separate script glxenv.sh for the common environment variables used by
the other GLX tests.
Removed the testx11glvndproto and testglxnscreens tests, since both of them
depend on the x11glvnd X extension, which is no longer used.
Removed the unused GLX_dummy_copy stuff from the GLX_dummy makefile. Also
removed the COMMON_* variables, since they're only ever used once each.