Stack frames: Better edge sources

[dwhatley19]

http://web.mit.edu/dwhatley/www/snapdown/web/production.html#%231%20-%3E%206%0Amain()%20%7Ba%20-%3E%20%231%7D%0Amain2()%20%7Ba%20-%3E%20%231%7D

Current result:

Edges should start "within" stack frames, i.e. right next to the variable. A solution would be to find the y-coordinate of the variable, and set walkable appropriate coordinates in the pathfinding grid.