A first attempt to make a simple and clean way of an on the fly videomode change.

A first attempt to make a simple and clean way of an on the fly videomode change.
But there is this problem:
it seems to work on the first transition from SDL to OpenGL, but doesn't work well in the following transitions to OpenGL.
I think it is not really related with the texture code, but with the st_video_setup_gl(). The way OpenGL is initialized.
Ingo/Tobias, please give a look at this ;)

SVN-Revision: 507