background_color is misused on color displays

[Liamolucko]

For some reason, all of the color displays seem to be using their background color as the color to set the chromatic layer to when rendering in black-and-white. This means that if their background color is set to black, the whole display gets set to red. (Also, after doing that by accident, it seems to have permanently washed out my display's red color; it might be because it was trying to set a pixel to black and red at the same time?)
On top of that, a lot of them don't actually use it as the color to clear the display in clear_frame either, just using the default background color.

As an aside, background_color seems a bit odd in general; it's only used for clear_frame, and doesn't actually affect the background color when rendering with embedded-graphics. Is it really needed?

[caemor]

The idea was to be able to change the background_color while running your program (with set_background_color) but it makes more sense to clean this up by removing this feature and delegate the drawing part to embedded_graphics and the user where needed.