Move TEST_SPRITESHEET to Constants
Add Segments When A Segment is Drug

WARNING: This can totally screw the game up for now because there are no
limits.
Bound Mouse Coordinates to the Grid
Limit Cables to Length Remaining

Of course, when you cut them short, things get weird right now.
Allow Puzzle State to Exfoliate the Grid
Allow Backtracking
Prevent Crash when Backtracking Too Far
Prevent Cables from Becoming Discontiguous
Fix Misaligned Sprite

The horizontal flat cable was one pixel off.