lavd/xcbgrab: do not try to create refcounted packets.

The framework will allocate a buffer and copy the data to it,
that takes time. But it avoids constently creating and
destroyng the shared memory segment, and that saves more time.

On my setup,
from ~200 to ~300 FPS at full screen (1920×1200),
from ~1400 to ~3300 at smaller size (640×480),
similar to legacy x11grab and confirmed by others.

Plus, shared memory segments are a scarce resource,
allocating potentially many is a bad idea.

Note: if the application were to drop all references to the
buffer before the next call to av_read_frame(), then passing
the shared memory segment as a refcounted buffer would be
even more efficient, but it is hard to guarantee, and it does
not happen with the ffmpeg command-line tool. Using a small
number of preallocated buffers and resorting to a copy when
the pool is exhausted would be a solution to get the better
of both worlds.
