GPencil: Fix T42895 - refactor object/scene data behaviour
- fix logic in ED_gpencil_data_get_pointers_direct
- make sure active scene greace pencil layers are always drawn
plus some picky whitespace cleanups
I stumbled over a few other minor gpencil things I'd like to fix before the release, will look at them soon.
(@Joshua Leung (aligorith), sorry for working on your baby, but it is kind of fun ;) )