GetFragDataLocation(prog, "gl_FragColor") generates INVALID_OPERATION, but specs don't say it should
We ran into this via fuzzers running on Firefox's WebGL implementation: https://bugzilla.mozilla.org/show_bug.cgi?id=1657189
I couldn't find any spec text that suggests INVALID_OP is valid for this case, either in GL3.3, GL4.6, or ES3.0.