Previously, when a texture atlas was destroyed we teared down
the  ImplOpenGLTexture even if there were OpenGLTexture instances
around. This caused that we could try to access an already
deallocated ImplOpenGLTexture which causes a seg. fault.

Now we change this so that a FixedTexture is no different than a
OpenGLTexture - we just release the reference, so any existing
OpenGLTextures for our texture would still be valid.

An additional problem is that FixedTexture registers a callback
for slot deallocation so we know when a OpenGLTextures that holds
a specific "slot" on the texture is deallocated. However if
FixedTexture is not existent anymore, the callback still gets
triggered and is trying to access invalid memory. To solve this
we need to unregister callbacks before FixedTexture is destroyed.

Additionally improve validity of a OpenGLTexture is valid. If
ImplOpenGLTexture is not allocated (nullptr) is one case, but in
addition to that if ImplOpenGLTexture has an id == 0 it also means
that it is not valid (anymore).

Change-Id: I87346198e8928e112619da62687d5856cb8aafb8