Support 2 / 3 HUB75 connectors

[jepler]

.. such as the "active 3" adapter from https://github.com/hzeller/rpi-rgb-led-matrix/tree/master/adapter/active-3

WIP branch: https://github.com/adafruit/Adafruit_Blinka_Raspberry_Pi5_Piomatter/compare/multiconnector?expand=1 -- this may or may not successfully blast out 3x as many pixels (it probably just hits an assertion error), but work in the pixel mapper is also needed.

[jepler]

rpi-rgb-led-matrix has a bunch of "mappers". These include some that deal with multiple connectors. We should study what they do
https://github.com/hzeller/rpi-rgb-led-matrix/blob/552ef44edf80fd56955c6e87ac322eda44b5f07c/examples-api-use/README.md#standard-mappers

also I think we should consider moving the mapping from C++ code to Python code, for ease of development. This would entail some other steps of re-organization, like turning the module into a package.