client/simple-dmabuf-v4l: Add fullscreen support

Effectively making the app behave very similar to a simple video player.
It was useful to me for testing hardware plane YUV support, so it's
likely worth upstreaming.

If available, wp_viewporter is used to scale up the image. The width to
height ratio is kept, allowing to further test black bars in fullscreen
mode.

For now scaling is only applied if xdg-shell is used.

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