- Replace BKE_brush_getactive_gpencil -> BKE_paint_brush
(no need for per-paint-type brush access).
- Rename TOT_GP_EDITBRUSH_TYPES -> GP_EDITBRUSH_TYPE_MAX
(avoid sharing prefix w/ unrelated constants).
- Rename instances of `GP_EditBrush_Data` to 'gp_brush'
(`Brush` is typically called 'brush').