protocol: add sub-surfaces

protocol: add sub-surfaces

Add protocol for sub-surfaces, wl_subcompositor as the global interface,
and wl_subsurface as the per-surface interface extension.

This patch is meant to be reverted, once sub-surfaces are moved into
Wayland core.

Changes in v2:

- Rewrite wl_subcompositor.get_subsurface description, and move mapping
  and commit details into wl_subsurface description. Check the wording
  in wl_subsurface.set_position description.

- Add wl_subsurface.set_commit_mode request, and document it, with the
  commit_mode enum. Add bad_value error code for wl_subsurface.

- Moved the protocol into Weston repository so we can land it upstream
  sooner for public exposure. It is to be moved into Wayland core later.

- Add destroy requests to both wl_subcompositor and wl_subsurface, and
  document them. Experience has showed, that interfaces should always
  have a destructor unless there is a good and future-proof reason to not
  have it.

Changes in v3:

- Specify, that wl_subsurface will become inert, if the corresponding
  wl_surface is destroyed, instead of requiring a certain destruction
  order.

- Replaced wl_subsurface.set_commit_mode with wl_subsurface.set_sync and
  wl_subsurface.set_desync. Parent-cached commit mode is now called
  synchronized, and independent mode is desynchronized. Removed
  commit_mode enum, and bad_value error.

- Added support for nested sub-surfaces.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>