evas-wayland-shm: Improve next buffer selection algorithm

evas-wayland-shm: Improve next buffer selection algorithm

When triple buffering it's possible that we'll only need two buffers at
a time for long durations.  When we finally call upon a third buffer it
hasn't been used recently enough to do a partial redraw.
By picking the oldest available buffer when multiple buffers are free we
can increase the likelihood of doing partial redraws.

Based on 79409757c6493738c21015708a1ba5178942ad94 by Derek Foreman.

Signed-off-by: Chris Michael <cp.michael@samsung.com>