Metal printer doesn't output gl_FragData correctly

[bkaradzic]

I had to add this code to fix metal printer:
bkaradzic@22be3e7

        if (strstr(var->name, "gl_") == var->name)
        {
            if (NULL == strstr(var->name, "gl_FragData_") ) {
                if (!var->data.used && !var->data.assigned)
                    continue;
            }
        }

The problem is that there are vars gl_FragData, and gl_FragData_0/1/etc.. gl_FragData is used and assigned, but gl_FragData_0/1/etc are not. In that case xlatMtlShaderOutput structure is just left empty causing error when building with metal compiler.