Tilemap editor mirror line

Summary

We should add toggle-able mirror lines on the x and y axis that will cause any tiles painted to be echoed on the other half.

Particularly when making arcade multiplayer maps, having a symmetrical tilemap is highly desirable. As tilemap size grows, however, this is hard to maintain manually. Mirroring should be an effect that can be turned on without affecting the current contents of the tilemap so that non-symmetrical elements are supported. Only elements painted while the mirror line was active would be mirrored.

Analysis

• Greatly speeds up level creation, particularly with the use to the alt-click and drag rectangular clone support (love the feature already).
• Would have to resolve odd-count tilemap indices. Probably just isolate the center row/column.
• Should it be positional to allow localized mirroring?