[clutter-texture] Use a GThreadPool for loading async textures

[clutter-texture] Use a GThreadPool for loading async textures

Instead of creating a separate thread for each texture load, the loads
are now queued in a thread pool which will use at most 3 textures at a
time. This avoids the potentially large overhead of creating and
destroying a thread for every image while still avoiding throttling
the CPU if a large number of textures are loaded at the same time.

If the load is cancelled the old code needed to join the thread which
effectively meant it had to wait for the load to finish. With this
model the threads are owned by the thread pool so there is no need to
wait for them to finish. Instead, each thread is given its own thread
data which we can use to mark the thread as aborted. Once the load is
finished the thread will check whether it is aborted before setting
the texture data. That way the ClutterTexture can just disown threads
that it needs to cancel.