minor update: undo!

Added

• Undo feature. An undo button is placed above the brush scaling UI.
  • You can undo the most recent 10 operations, including “Clear”s and “Load”s. Resize is not undo-able yet.

Changed

• Brush scaling is in increments of 1 pixel now. Previously it was increments of 2, meaning that for low res canvases you were constrained to drawing 2x2 instead of having fine single pixel control.