kwin: Resolve functions for GL_ARB_robustness

Only the subset of functions available in core contexts is resolved,
except for glGetnTexImageARB() and glGetnUniformivARB(), which are
not used by kwin.

Instead of setting the function pointers to NULL when the extension isn't
supported, kwin provides its own implementations that call the non-robust
versions of the functions. This is so callers don't have to check if the
extension is supported before calling the functions.