protocol: add sub-surfaces to the core

The sub-surface protocol was originally committed into Weston on May
10th, 2013, in commit 2396aec6842c709a714f3825dbad9fd88478f2e6. The
design for the protocol had started in the beginning of December 2012. I
think it is high time to move this into the core now.

This patch copies the sub-surface protocol as it was in Weston on Nov
15th, 2013, into Wayland. Weston gets a patch to remove the protocol from
there.

Sub-surface is a wl_surface role. You create a wl_surface as usual, and
assign it the sub-surface role and a parent wl_surface. Sub-surfaces are
an integral part of the parent surface, and stay glued to the parent.
For window management, a window is the union of the top-level
wl_surface and all its sub-surfaces. Sub-surfaces are not clipped to the
parent, and the union of the surface tree can be larger than the
(top-level) wl_surface at its root.

The representative use case for sub-surfaces is a video player window.
When the video content is given its own wl_surface, there is no need to
modify the video frame contents after decoding or copy them into a whole
window sized buffer before submitting it to the compositor. This allows
efficient, zero-copy video presentation paths, where video decoding
hardware produces a (YUV) buffer, which eventually ends up in a
(YUV-capable) hardware overlay and is scanned out directly.

This can also be used for zero-copy presentation of windowed OpenGL
content, where the OpenGL rendering engine does not need to draw or
avoid window decorations.

Sub-surfaces allow mixing different buffer types into the same window,
e.g. software-rendered decorations in wl_shm buffers, and live content
in EGL-based buffers.

However, the sub-surface extension does not offer clipping or scaling
facilities, or accurate presentation timing. Those are topics for
additional extensions.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>