D18465: [platforms/wayland] Multi output support

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REVISION SUMMARY
  This patch rewrites large parts of the Wayland platform plugin, in order to
  facilitate the testing of multi output behavior in nested KWin sessions.
  
  For that a new class WaylandOutput is introduced, which is based on
  AbstractOutput and by that shares functionality with our virtual and DRM
  platform plugins.
  
  The EGL/GBM and QPainter backends have been remodelled after the DRM one,
  sharing similiarities there as well now.
  
  Pointer grabbing has been rewritten to support multiple outputs, now using
  pointer locking instead of confining and drawing in this case onto a sub-
  surface, which get dynamically recreated in between the different output
  surfaces while the cursor is being moved.
  
  Window resizing is possible if host supports xdg-shell, but currently the
  mode size does not yet fill the new window size.
  
  The number of outputs can be set by command line argument `--output-count`,
  scaling is also supported by setting the argument `--scale`.
  
  Further steps could be:
  
  - Enabling automatic fill of resized windows via Wayland mode change
  - Multiple diverging initial sizes and scale factors for mulitple outputs

TEST PLAN
  Tested it in live session.

REPOSITORY
  R108 KWin

BRANCH
  waylandBackendMultiOutput

REVISION DETAIL
  https://phabricator.kde.org/D18465

AFFECTED FILES
  plugins/platforms/wayland/CMakeLists.txt
  plugins/platforms/wayland/egl_wayland_backend.cpp
  plugins/platforms/wayland/egl_wayland_backend.h
  plugins/platforms/wayland/scene_qpainter_wayland_backend.cpp
  plugins/platforms/wayland/scene_qpainter_wayland_backend.h
  plugins/platforms/wayland/wayland_backend.cpp
  plugins/platforms/wayland/wayland_backend.h
  plugins/platforms/wayland/wayland_output.cpp
  plugins/platforms/wayland/wayland_output.h

To: romangg, #kwin
Cc: kwin, jraleigh, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, ngraham, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart