Changes for opengl render to reflect new alpha premul pipeline
without hurting quick texture painting

- ED_view3d_draw_offscreen will now output buffer with
  transparent alpha, if sky needed it should be alpha-undered
  later.

- ED_view3d_draw_offscreen_imbuf now accepts alpha mode as an
  argument which could be either R_ADDSKY or R_PREMULALPHA

- OpenGL render and sequencer's opengl preview will now reflect
  scene's Alpha Mode

- Quick Edit will use OpenGL with transparent alpha mode

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53588 954f8c5b-7b00-dc11-b283-0030488c597c