When allocate_display_bus_or_raise was factored out, the assignment
of the bus's Python type was lost. Restore it.
This would have affected displays of any type other than RGBMatrix, when
they were created dynamically. Boards with displays configured in flash
were unaffected.
Closes: #2792
This also improves Palette so it stores the original RGB888 colors.
Lastly, it adds I2CDisplay as a display bus to talk over I2C. Particularly
useful for the SSD1306.
Fixes#1828. Fixes#1956