Replace embedded-graphics dependency

[kpreid]

While embedded-graphics has been a useful source of drawing operations, there are disadvantages:

• In the next release, PixelColor implementers are going to be required to be convertible from bytes which means we'll need to indirect through a palette to draw any blocks that aren't plain color blocks
• The main reason I originally used it at all was text rendering with built-in fonts, but in the long run I want user-defined fonts, built-in fonts with higher coverage, and 3D text (see Text rendering #424).
• We still haven't succeeded at fully correct e-g-2D/aic-3D coordinate and bounding box conversion under arbitrary transforms. This is not fundamentally hard; it's just easy to get wrong.
• Using 2D drawing primitives discourages us from taking advantage of 3D.
• Aesthetic complaint: Docs get cluttered with all of the blanket traits from az.

Therefore, I believe we should replace e-g. Its current usage is:

• Rendering:
  • Info text overlay
  • Writing pixels to the image in raytrace_to_texture
• Text in the world:
  • Evaluating Primitive::Text.
  • Drawing text inside make_some_voxel_blocks() (601910d)
  • Drawing text on demo-city exhibit signs
  • Drawing buttons' text “icons” (af2835d)
  • vui::widgets::LargeText
• Non-text content:
  • Button widget's base shapes
  • Icons::Delete
  • Colored lights exhibit's test card pattern
  • Copying loaded image assets into blocks.

Migration plan:

1. Implement raytrace_to_texture directly (perhaps with the help of imgref), without e-g involved.
2. Change button icon text to be real icons or text labels.
3. Replace Icons::Delete with an image file.
4. Replace light test card with an image file.
5. Replace button shapes with image files.
6. Use Primitive::Text for make_some_voxel_blocks().
7. Replace all direct text rendering with Primitive::Text or (for large text) a way to ask our own fonts to draw into a SpaceTransaction or for their pixels (for info text overlay).
8. Delete (or make private) draw_target() methods.
9. Replace implementation of text rendering (with what?).
10. Remove embedded-graphics entirely.

Open questions:

• Should our new text renderer be from scratch or is there another library that we can use? I don't have notes from the previous library search.