* Grease Pencil works again from Image Editor now. For now, the GPencil datablock is linked to the Image Editor space, but this can be changed if need be.
* Made Grease Pencil hotkeys into a separate Grease Pencil keymap, which can get included automagically like for frames/ui/v2d/etc. by supplying ED_KEYMAP_GPENCIL as part of st->keymapflag
* Temporarily restored the nasty hack to make View2D-aligned sketches in Image Editor to use OpenGL lines only. I still dunno why this doesn't work normally.
(Probably related is that strokes are not visible when there's no image visible atm).
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