D23669: Generate mipmaps for image down-scaling

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REVISION SUMMARY
  Without generating mipmaps the output of via glViewport down-scaled images
  is blurry when not being scaled by an integer value.

TEST PLAN
  Tested on an Intel setup where the image is fine afterwards. On an AMD setup
  I have a crash in mesa (experimental mesa ACO branch).

REPOSITORY
  R108 KWin

BRANCH
  mipmapScaling

REVISION DETAIL
  https://phabricator.kde.org/D23669

AFFECTED FILES
  platformsupport/scenes/opengl/abstract_egl_backend.cpp
  plugins/scenes/opengl/scene_opengl.cpp

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