mel rotomap-compare: keep per-rotomap cursor
mel rotomap-compare: keep per-rotomap cursor

When cycling through rotomaps, remember the selected image for each
rotomap. This is to support the workflow of selecting the best image for
each rotomap, and cycling through the rotomaps to compare.

Test Plan:
.. Compare a few rotomaps ..

aevri/mel,aevri/mel