Create singleton to store pan and scale values

[n8rzz]

The pan values currently live within the CanvasController and the scale value lives within the UiController. This is confusing and should be consolidated.

These values should live within their own class which would provide getters and setters and some translation methods. The classes using these values can then reference the singleton as a single source of truth.

should store:

• canvas.size.width and canvas.size.height
• panX and panY
• scale

[n8rzz]

work on this should happen after #707 is merged due to the likelihood of massive merge conflicts