Update glext.h to a version that isn't four years old

Originally committed to SVN as r4454.

Update glext.h to a version that isn't four years old
303f4990 · Thomas Goyne · ad0241e3 · 303f4990 · 303f4990
--- a/aegisub/LICENCE
+++ b/aegisub/LICENCE
@@ -43,7 +43,7 @@ src/boost/
 - Boost Software License Version 1.0 see src/boost/LICENSE_1_0.txt.

 src/gl/
- - SGI Free Software License. See src/gl/glext.h
+ - MIT license. See src/gl/glext.h

 src/MatroskaParser.(c|h)
 - Licensed to BSDL with permission from the author.

--- a/aegisub/src/gl/glext.h
+++ b/aegisub/src/gl/glext.h