Use the correct macro for GLX_CONTEXT_PROFILE_MASK_ARB
Use the correct macro for GLX_CONTEXT_PROFILE_MASK_ARB

GLX_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB is the correct counterpart to
GLX_CONTEXT_CORE_PROFILE_BIT_ARB in this context.  This has the same value as
the GLX_CONTEXT_FORWARD_COMPATIBLE_BIT_ARB it replaces though, so this change
should not actually affect anything.

drobilla/pugl,drobilla/pugl,drobilla/pugl