gimp_airbrush_stamp(), which is called during the airbrush timeout
to periodically stamp the airbrush, assumes the saved symmetry's
origin is the same as during the original call to
gimp_airbrush_paint().  However, since commit
bc09c718, we clear the symmetry's
origin at the end of gimp_paint_core_paint(), resulting in an empty
symmetry with a NULL origin during gimp_airbrush_stamp().  As a
result, no dab is painted, and we segfault if there's active
dynamics.

Fix this by saving the symmetry's origin coords during
gimp_airbrush_paint(), and restoring them in gimp_airbrush_stamp().