Refactor: Coordinate system

[amclain]

Depends on #43.

As mentioned in the 2020-06-23 meeting, the Keybow's physical pixels are rendered based on a position-dependent list, @pixels, in utils.ex. In this list the elements have to be in the same order that the pixels are arranged on the Keybow on the SPI bus. We would like hardware layer that controls the pixels (RGBMatrix) to be responsible for translating the pixels in a Frame to the correct physical representation.

TBD: The strategy we choose to refactor this will most likely be dependent on how we decide to design pixel layouts in #43.