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function: goto_point -- goto saved vertex location
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keyboard equiv Linux: <Alt-g>
keyboard equiv Mac: <Cmd-g>

The "GOTO PNT" button reads coordinates in the
file $SUBJECTS_DIR/<subj>/tmp/edit.dat, which
contains a vertex location written by "SEND PNT"
(from either tkmedit, tkregister, or tksurfer).

The pixel corresponding to the saved point is
located and the cursor is put there (current
plane unchanged).
