Some operators like curve presets, color sample and some more were using object's
mode to distinguish in which mode user is currently painting. Such approach fails
in cases when there's paint mode active in 3D viewport and Image Editor.
Changed logic here to use some context's state like active space which helps
distinguishing current paint mode more accurate.
Ported all areas which uses paint_get_active() to new paint_get_active_from_context().
There're still some calls to paint_get_active(), but that shouldn't be harmful due
to that places indeed have object's mode as priority when getting paint mode.
* Renamed flip_coord as flip_v3_v3
* Added flip_v3 for same input/output
* Moved special case for grab brush's normal into calc_area_normal()
* Renamed 'fixed' texture mode as 'view plane', mirrors
Brush.sculpt_plane terminology
The paint cursor code is fairly muddled still and needs futher cleanup
(commented in the new file.) Over half the paint_stroke code was just
called from the cursor draw function.
There should be no functional changes from this.
