todo.txt

GUI Improvements
- A layer widget
- A new history widget, or making the TreeView better
- General pretty-ing

Unimplemented Tools
- Zoom
- Magic Wand
- Pan
- Paint Bucket
- Gradient
- Clone Stamp
- Recolor
- Text
- Line/Curve

Unimplemented Adjustments
- Brightness/Contrast
- Curves
- Hue/Saturation
- Levels
- Posterize

(Note that the adjustments are pretty easy to implement by reusing
Paint.NET's code.  The bigger job is writing the Gtk dialogs.)

Port Paint.NET's Effects
- None have been ported yet.

History
- Persist some history to disk
- Don't store entire surfaces when not needed
- Preferences dialog to control how much history to keep,
  and where (memory/disk)

Performance
- For ease of implementing, we almost always Invalidate the entire
  canvas.  We need to track what we actually change and only invalidate
  the dirty area.