Scaling and zooming controls

[cdsmith]

For picture, animation, and simulation modes, add controls for the students to zoom and pan the canvas. Gloss has these, and they are often convenient.

[cdsmith]

@kammitama5 I was just looking through old bugs, and ran into this. As long as you're working on debugger controls, this is another possibility to consider. Basically, you'd apply a pan (translation) and zoom (scaling) to the drawing, and add controls to adjust it. Maybe just any dragging on the screen would pan, and separate zoom-in and zoom-out buttons could be used to zoom. Any time it's non-zero, you could show a button to reset to the default view.

I don't want to distract you, but if you're sick of time traveling and want to work on something easier some night, this is an option.

[cdsmith]

Fixed by #701

Thanks @kammitama5