Changed the painting view to construct a string of pixel rep on fly.

Instead of tracking the touches and worrying about it later, painting
view will keep a dynamic string of 1s and 0s representing the places
with ink.
Bug, the string changes to immutable after 2 rounds.

Don't know why, but I cast mutable every time now.
Doesn't crash at least.

It works... sometimes. Something to go with but not
real impressive.