Optimize minimap updates

[bjorn]

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.