clients/simple-egl: Handle buffer scale and transform

Buffer scale is common enough in the modern desktop space to
expect average GL clients to handle it. Thus lets include it into
our main example client.

While on it, also handle buffer transforms. It's essentially free
for GL clients in terms of computing power but may increase the
chance that Wayland compositors are able to hit scanout fast paths.
Thus having an example client for it is likely valueabel for client
and compositor developers.

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