Inspect: Visual debugging aids

When students enable "Inspect" on their programs, we want to:

1. Add a coordinate plane to the picture.
2. Convert all solid shapes to opaque outlines with translucent fill. This allows students to see when another shape is hidden behind this one.

This was inspired by @alphalambda who adds a special debug version of `drawingOf` to LSU's fork, which implements something like both of these changes. Instead of a different entry point, though, I want to do it by just toggling debug mode in the same running program.