glxwrap: Initialize fips_dispatch when glxMakeContextCurrent is called

Previously, we only intitialized fips_dispatch if glxMakeCurrent was
called.  This caused fips to fail for programs that called
glxMakeContextCurrent instead. Both functions are now handled
indentically, (giving fips a clear indication that GLX is being used,
not EGL).

This fixes the failure of fips with Lightsmark 2008.