Native bindings to the EXT_config_select_group extension.

This extension provides a mechanism to lower the config selection priority of the configs returned by ChooseConfig.

A new config attribute is introduced, providing a way for the implementation to group configs into different config selection categories. The config selection priorities of all the configs in one category may be higher or lower than the ones in other categories. The config selection priorities in one category follow the rules of eglChooseConfig. See also GLX_SGIX_visual_select_group.