Added support for bitdepths in OnDiskBitmap
Added support for bitdepths in OnDiskBitmap

https://github.com/adafruit/circuitpython/pull/1558
adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython