clients/simple-dmabuf-v4l: Add pointer support

To make the app a little bit more pleasant to work with and, crucially,
makes the cursor hide in fullscreen mode. This is useful when testing
hardware planes on devices with no cursor plane / a single primary plane.

Also add an option to keep the cursor around in fullscreen mode.

Signed-off-by: Robert Mader <robert.mader@collabora.com>