Eraser Tool, Brush Sizes

* Implement Brush Sizes for drawtool.Stroke and add a UI to the tools panel
  to control the brush size.
  * Brush sizes: 1, 2, 4, 8, 16, 24, 32, 48, 64
* Add the Eraser Tool to editor mode. It uses a default brush size of 16
  and a max size of 32 due to some performance issues.
* The Undo/Redo system now remembers the original color of pixels when
  you change them, so that Undo will set them back how they were instead
  of deleting the pixel entirely. Due to performance issues, this only
  happens when your Brush Size is 0 (drawing single-pixel shapes).
* UI: Add an IntVariable option to ui.Label to bind showing the value of
  an int reference.

Aforementioned performance issues:

* When we try to remember whole rects of pixels for drawing thick
  shapes, it requires a ton of scanning for each step of the shape. Even
  de-duplicating pixel checks, tons of extra reads are constantly
  checked.
* The Eraser is the only tool that absolutely needs to be able to
  remember wiped pixels AND have large brush sizes. The performance
  sucks and lags a bit if you erase a lot all at once, but it's a
  trade-off for now.
* So pixels aren't remembered when drawing lines in your level with
  thick brushes, so the Undo action will simply delete your pixels and not
  reset them. Only the Eraser can bring back pixels.

interface.go

@@ -254,6 +254,7 @@ func IterRect(p1, p2 Point) chan Point {
 				X: TopLeft.X,
 				Y: BottomRight.Y,
 			}
+			dedupe = map[Point]interface{}{}
 		)
 
 		// Trace all four edges and yield it.
@@ -268,7 +269,10 @@ func IterRect(p1, p2 Point) chan Point {
 		}
 		for _, edge := range edges {
 			for pt := range IterLine2(edge.A, edge.B) {
-				generator <- pt
+				if _, ok := dedupe[pt]; !ok {
+					generator <- pt
+					dedupe[pt] = nil
+				}
 			}
 		}