function mightBeValidEnum(fname, i, value) {
       if (!mightBeEnum(value)) return false;
       return (fname in glValidEnumContexts) && (i in glValidEnumContexts[fname]);
}
Returns true if 'value' matches any WebGL enum, and the i'th parameter
of the WebGL function 'fname' is expected to be (any) enum. Does not
check that 'value' is actually a valid i'th parameter to 'fname', as
that will be checked by the WebGL implementation itself.

@param {string} fname the GL function to use for screening the enum
@param {integer} i the parameter index to use for screening the enum
@param {any} value the value to check for being a valid i'th parameter to 'fname'
@return {boolean} true if value matches one of the defined WebGL enums,
        and the i'th parameter to 'fname' is expected to be an enum

@author Tomi Aarnio