Add a mechanism to create, recycle and destroy window buffers when
needed.
Typically, this adds a new `xwl_window_buffer` structure which
represents a buffer for a given Xwayland window.
Each Xwayland window has two different pools of buffers:
- The available buffers pool:
Those are buffers which where created previously and that have either
not been submitted to the compositor or submitted and released.
- The unavailable buffers pool:
Those are typically the buffers which are being used by the
compositor, awaiting a release.
Initially, an Xwayland window starts with both pools empty. As soon as a
new buffer is needed, it's either created (if there is none available)
or picked from the pool of available buffers.
Once submitted to the compositor, the buffer is moved to the pool of
unavailable buffers. When the corresponding `wl_buffer` is released by
the compositor, it is moved back to pool of available buffers again to
be reused when needed.
To avoid keeping too many buffers around doing nothing, a garbage
collection of older, unused buffers also takes care of disposing the
buffers being unused for some time.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
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