Optimize minimap updates

Nobody has claimed this yet.

When editing large maps the minimap is not performing well enough, causing annoying pauses while the minimap is repainting. Possible fixes:
- Move the updating of the minimap into a thread. This would require the map data and used tilesets to be cloned to prevent changes to it to crash the minimap rendering thread.
- Only redraw the actually changing areas. For most operations this would make the minimap updates really quick since they tend to affect only a small area.

The only available workaround to this problem currently is to close the minimap, which suppresses the repaints.

Start by locating the minimap repaint and update path in the editor, then reproduce the pauses with a large map. Compare the issue's two proposed directions—threaded updates or redrawing only changed areas—and establish measurements for the current behavior. Done means large-map editing no longer causes the reported minimap pauses, with the chosen approach verified against relevant editing operations.

Written by the indexing model from the issue text.