FIX: renamed VistaTexture to VistaGLTexture in order to prevent name clash with …

…VistaTexture from VistaOGLExt

It would be better if the two texture classes could be merged into one. However, this commit fixes the problem temporarily. If you do not like the name VistaGLTexture you can easily replace it with something different. The complicated part was to rename the correct occurences of VistaTexture :)