glext too big

[nlguillemot]

right now the header has the entire contents of glext.h

over 90% of what glext has is useless old stuff. Should remove it all while keeping the few things that are still useful these days (like anisotropy, compressed texture formats...)

Not only does it make the header way way bigger, but also lowers the usefulness of autocomplete in editors by adding a bunch of pointless functions.