Slot mechanism in ImplOpenGLTexture was written to support needs
for FixedTextureAtlas. This commit makes the slot mechanism more
general so it can be used in other kinds of texture atlases like
PackedTextureAtlas.

The ImplOpenGLTexture still tracks slots, but it is not needed to
define beforehand how many slots there are. The deallocation has
been factored out, ImplOpenGLTexture instead calls a callback
function that a slot with a specific "slot id" has been
deallocated.

Change-Id: I23950d325b803969f958d03ebf34805687c4e620