As part of rB3f91540cef7e, we already made OB_MODE_SCULPT_CURVES to be
allowed in paint_curve_poll (alongside OB_MODE_ALL_PAINT).
Now, to get the paintcurves transform systems to work with curves
sculptmode as well, we introduce this "additional case" in the
appropriate place in the transform system as well.
NOTE: as a next step, considering OB_MODE_SCULPT_CURVES to be
generally part of OB_MODE_ALL_PAINT is to be done (this might fix
another couple of bugs, but also has to be carefully checked in many
places, so this patch is just fixing this very specific case)
Fixes T102204.