D25903: Cache output transformation first

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REVISION SUMMARY
  First we need to set a transformation, then update the backend. In DRM the
  cursor gets updated and needs the new transformed size.
  
  Longterm we need a pending state if we want to apply configuration changes
  atomically or deny them.

TEST PLAN
  Compiles.

REPOSITORY
  R108 KWin

BRANCH
  transform-set

REVISION DETAIL
  https://phabricator.kde.org/D25903

AFFECTED FILES
  abstract_wayland_output.cpp

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