Merge branch 'stage-use-alpha' into ebassi-next

* stage-use-alpha:
  tests: Use accessor methods for :use-alpha
  stage: Add accessors for :use-alpha
  tests: Allow setting the stage opacity in test-paint-wrapper
  stage: Premultiply the stage color
  stage: Composite the opacity with the alpha channel
  glx: Always request an ARGB visual
  stage: Add :use-alpha property
  materials: Get the right blend function for alpha

diff --cc clutter/clutter-stage.h
Simple merge

diff --cc doc/reference/clutter/clutter-sections.txt
Simple merge