texture: Expose subclasses as subclasses

texture: Expose subclasses as subclasses

This is necessary so that bidnings work properly and don't make
gdk_gl_texture_release() a function on GdkTexture.
It also allows code to identify what type of texture they are dealing
with.

Finally, we can now decide to add getters later without screwing
anything up, if we want to allow people to access GL textures directly.