clients: add subsurfaces demo

clients: add subsurfaces demo

Add a demo program with:
- a main surface (green)
- a Cairo-image sub-surface (red)
- a raw GLESv2 widget (triangle)

Sub-surface input region is set empty to avoid problems in toytoolkit.

If Cairo links to libGL, then we will end up with also libGLESv2 linked
to subsurfaces program, and both libs getting really used, which leads
to disaster.

Do not build subsurfaces demo, if Cairo links to libGL and cairo-egl is
usable.

The GL rendering loop is not tied to the toytoolkit or the widget, but
runs directly from its own frame callback. Therefore it runs
independent of the rest of the application. This also relies on one of
two things:
- eglSwapInterval(0) is implemented, and therefore eglSwapBuffers never
  blocks indefinitely, or
- toytoolkit has a workaround, that guarantees that eglSwapBuffers will
  return soon, when we force a repaint on resize.
Otherwise the demo will deadlock.

The code is separated into three sections:

1. The library component, using only EGL, GLESv2, and libwayland-client
   APIs, and not aware of any toolkit details of the parent application.
   This runs independently until the parent application tells otherwise.

2. The glue code: a toytoolkit application widget, who has its own
   rendering machinery.

3. The application written in toytoolkit.

This patch also adds new toytoolkit interfaces:
- widget_get_wl_surface()
- widget_get_last_time()
- widget_input_region_add()

Toytoolkit applications have not had a possibility to change the input
region. The frame widget (decorations) set the input region on its own
when used, otherwise the default input region of everything has been
used. If a window does not have a frame widget, it can now use
widget_input_region_add() to set a custom input region.

These are not window methods, because a widget may lie on a different
wl_surface (sub-surface) than the window.

Changes in v3:
- replace set_commit_mode with set_sync and set_desync

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