Add support for external objects

[scheibel]

OpenGL registry entries:

• https://khronos.org/registry/OpenGL/extensions/EXT/EXT_external_objects.txt
• https://khronos.org/registry/OpenGL/extensions/EXT/EXT_external_objects_fd.txt
• https://khronos.org/registry/OpenGL/extensions/EXT/EXT_external_objects_win32.txt
• https://khronos.org/registry/OpenGL/extensions/EXT/EXT_win32_keyed_mutex.txt