* Move symmetry options to the paint struct (where all paint systems can
make use of it)
* Rename draw_pressure to stroke_active. This is what is really checked
on those occasions that this is used. Also move turning on/off of this
option to the stroke level and avoid doing it on every stroke system.
* Rename BRUSH_RESTORE_MESH to BRUSH_DRAG_DOT. In image painting this
won't restore any mesh, so better have a name that is directly linked to
what the flag actually does.
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