Add support for broadcom (raspberry pi)

[rsbohn]

The broadcom port display is a FramebufferDisplay.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "adafruit_bitmapsaver.py", line 127, in save_pixels
ValueError: Second argument must be a Bitmap or Display

If I disable that check I get this:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "adafruit_bitmapsaver.py", line 94, in _write_pixels
ValueError: Display must have a 16 bit colourspace.

The broadcom display uses a 32 bit colourspace.

[tannewt]

I think this will take additions to CircuitPython as well. It may be more expedient to use an HDMI capture dongle to capture the screen.