clients/simple-egl: Implement fractional-scale protocol support

Fractional scale is increasingly common in the Wayland ecosystem. Thus,
given simple-egl's role as egl example client, implement support for
the new protocol - even though Weston itself does not support it yet.

Together with buffer_scale and buffer_transform this ensures
simple-egl provides optimally sized and oriented buffers.

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