Brush cursors were missing when opening a file saved in sculpt/vertex-/
weightpaint mode.
Since we dont do a full modeswitch on file load in `ED_editors_init` for
grease pencil since rBde994d6b7b1c, we were missing the brush cursor
toggling [`ED_gpencil_toggle_brush_cursor`] normally done in
`ED_gpencil_setup_modes`.
This is now explicitly added for any greasepencil paintmode (in case
object is active).
Maniphest Tasks: T102685
Differential Revision: https://developer.blender.org/D16603
179008f57a