[Impeller] Add support for swapping backends dynamically in impeller/typographer.

This will allow us to bring in and [test an STB based backend](https://github.com/bdero/impeller-cmake/commit/44035a6dd9403b10251b1c30bd8b78ee4187f460) that is easier to prototype with as it pulls in only a single header dependency.