GL loader work

[sfan5]

todo (not necessarily all here):

• [x] make extpointer mandatory
• [x] figure out why the first commit broke world rendering on gles2 ~~(not my fault?)~~
• [x] Drop unused exts from COGLESCoreExtensionHandler
• [ ] ~~Make COGLES2ExtensionHandler use loader~~
• [ ] ~~Make COGLES1ExtensionHandler use loader~~
• [ ] ~~Drop unused exts and functions from COpenGLExtensionHandler (big oof)~~
• [ ] ~~Make COpenGLExtensionHandler use loader~~
• [ ] Identify and migrate other GL calls
• [x] Deal with glext.h ?!
• [x] macOS support should automatically start working at this point

Edit: I crossed half out because I'm not actually sure what the way forward is. The GLES2 driver has some sort of unified stuff where the new support code uses GL instead of direct calls, while I assume GLES1 is meant to be dropped and GL to be replaced by GL3.