Each tool can have its own set of properties that can be modified
through a panel on the right side.
The tools I've added properties for are:
Pen:
Thickness
Brush:
Size
Hardness
Spray:
Thickness
Density
Bucket:
Threshold
Indexed bitmaps used to allocate four times the required amount of memory.
Also, we should acknowledge that the underlying data is not always RGBA32,
and instead cast it only when the true type is known.
We use this to automatically update the thumbnail in LayerListWidget
when you draw into a layer. We also use it to repaint the ImageEditor
when the image changes somehow. :^)
