clients/dmabuf-intel: use three buffers
clients/dmabuf-intel: use three buffers

Use three buffers like simple-dmabuf-v4l instead of just two.

This is required, because when a frame callback arrives, the just
committed buffer is only on its way to the screen, while the previous
buffer is still being scanned out. It will take for the page flip to
complete, before the previous buffer is release. However, we want to be
able to repaint already at the frame callback, so three buffers can be
necessary.

This patch fixes weston-simple-dmabuf-intel to not abort with "Both
buffers busy at redraw()." when hardware overlays are used and the
surface gets directly scanned out.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-by: Yong Bakos <d0d6c2a9047d8c98cd8ce1e9b46eb851dcd10a82@humanoriented.com>

sir-murray/weston,sir-murray/weston,krezovic/weston,krezovic/weston