Improvements to the stretch tool

[shannonturner]

Suggestion from email:

Is it possible to make one able to stretch a 240x240 map (The previous max size) to a 360x360 map (The new max size)?

This might be difficult since stretching essentially doubles all points in a line, and while isMapStretchable() can be fudged to always return true, that would result in data loss on the right/bottom sides of the map.

One thing I could do would be to make isMapStretchable() calculate your current actually-used area rather than relying on gridRows / gridCols, which would allow you to stretch a map that was 180x180 (on a 200x200 canvas) to 360x360.

One thing that may help the requester is when #34 is released, since then a portion of the map could be stretched.