Made BackgroundColor public

[asajeffrey]

Trying to use BackgroundColor outside pathfinder_demo results in:

error[E0603]: enum `BackgroundColor` is private
  --> demo/magicleap/src/lib.rs:28:22
   |
28 | use pathfinder_demo::BackgroundColor;
   |                      ^^^^^^^^^^^^^^^

or

error[E0603]: module `ui` is private
  --> demo/magicleap/src/lib.rs:28:22
   |
28 | use pathfinder_demo::ui::BackgroundColor;
   |                      ^^

[pcwalton]

I have an informal policy of no pub use, because I think it makes crates harder to understand. I landed this without it in 782e352.