Support Pi4 second HDMI screen

[tannewt]

From @TheMindVirus on tannewt#14:

Hi Scott, not an issue as such but more of a feature suggestion.
Great work on the weekly streams for Adafruit, it shows people the kind of stuff we have to deal with as embedded developers.

I was looking at this file that you have for displayio in CircuitPython for Raspberry Pi SBC's:
https://github.com/tannewt/circuitpython/blob/rpi/ports/broadcom/bindings/videocore/Framebuffer.c
There might be more information I'm missing but that's what I could see.

It is of similar purpose to what I have been working on to support Pi's that have more than one framebuffer:
https://github.com/TheMindVirus/PiX-iES/blob/second-monitor/iES.C
It's based on some of the same bare-metal tutorials you mentioned along with my own code originally for my Windows Drivers.

I have some more repo's and branches which might be of use to the project. Pick and choose which bits you'd like to use or modify.
Looking forward to solving some more issues e.g. with TinyUSB, though the appropriate information is seldom made open source.