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