Enhancements for smoother zooming

[IonoclastBrigham]

Each canvas in Racket has a backing dc%, which can be used to set a scaling factor, rather than manually resizing the in-memory bitmap object. The result should be faster and smoother, and will set the stage for possibly switching to OpenGL rendering in the future.

• Remove all image scaling, replace with "dc-zoom"
• Clean up relevant codepaths along the way, to facilitate future maintenance and features
• Add more zoom options to the menu

[lehitoskin]

This was resolved some time ago. Never got around to closing this issue.

[IonoclastBrigham]

Resolved by 2c9b862.