Headers gl2.h and gl2ext.h shipped with NaCl are different to the
official ones, which is causing linker issues, thus using NaCl's own
gl2.h. They are otherwise similar, thus it should cause no compatibility
issues.
On the other hand, gl2ext.h shipped with NaCl is slightly outdated with
some recent extensions missing. We are including the NaCl's one and then
the official one over it (undefining the include guard). The symbols are
guarded also by extensions, so it should cause no conflicts.
Moved them to `OpenGL/` subdirectory, allowing them to be included
explicitly with e.g. <OpenGL/GLES2/gl2ext.h> overriding the system
<GLES2/gl2ext.h> header. Our versions of the headers are thus now
explicitly included in `OpenGL.h`, but they can be also included using
no-prefix path if no system version is available. It might break some ES
platforms, they will be fixed when found.
The headers are now installed into `Magnum/OpenGL` (not into any
artificial `external` directory). Now also installing GLES2 headers for
OpenGL ES 2 (previously ES3 headers were installed for both ES2 and
ES3).
Vladimír Vondruš