- Removed IImageCompressed and derived.
- Extended IImage class for support compressed textures.
- Improved DDS loader for support mip map loading.
- Added DXT1-5 compressed textures support for OpenGL.

git-svn-id: https://irrlicht.svn.sourceforge.net/svnroot/irrlicht/trunk@4449 dfc29bdd-3216-0410-991c-e03cc46cb475