Additionally:
- hide the GLTexture implementation using dpointers
- drop the unused function SceneOpenGL::Texture::optimizeBindDamage()
- Texture::load now loads a new texture and does not update the existing one
REVIEW: 101999
Now convertToGLFormat does no longer mirror the texture, so that we can
avoid a deep copy in case we render a GL_BGRA texture on a little endian system.
REVIEW: 101739
