Add support for broadcom (raspberry pi)

The broadcom port display is a FramebufferDisplay.

```
Traceback (most recent call last):
File "", line 1, in
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 "", line 1, in
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.