This is the only place where it is called.

There was also no need for this function to be virtual since it's not
reimplemented anywhere.

This also fixes the GLTexturePrivate being created only to be
immediately deleted and replaced by a new GLTexturePrivate when the
GLTexture(QImage) constructor was used.

With this change the structure of the GLTexture effectively becomes
immutable to the outside world; the contents of the texture images can
change, but the texture images themselves cannot be reallocated.