D26172: Add wp_viewporter support

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REVISION SUMMARY
  With the new interface in KWayland we can add support for viewports. Here the
  specified viewport is being respected by setting the texture coordinates when
  rendering with OpenGl and setting the source rectangle with QPainter.
  
  The destination size is already implicitly respected through the size of the
  KWayland SurfaceInterface.

TEST PLAN
  Tested with weston-scaler.

REPOSITORY
  R108 KWin

BRANCH
  viewporter

REVISION DETAIL
  https://phabricator.kde.org/D26172

AFFECTED FILES
  plugins/scenes/qpainter/scene_qpainter.cpp
  scene.cpp
  wayland_server.cpp
  wayland_server.h

To: romangg, #kwin
Cc: kwin, LeGast00n, The-Feren-OS-Dev, sbergeron, jraleigh, zachus, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, romangg, jensreuterberg, abetts, sebas, apol, ahiemstra, mart