rotomap-edit: Cache loaded image
rotomap-edit: Cache loaded image

Loading iamges from disk appears to be quite slow. We do this repeatedly
when zooming and re-drawing.

Cache the current image to avoid this cost when re-drawing the current
image.

Test Plan:
$ ./meta/run_tests.sh
.. View a single image and verify that it's noticably faster to zoom ..

aevri/mel,aevri/mel