MapMask is broken

[artyompal]

I'd like to report the following problems with MapMask:

1. It uses foreground=255, but if you look to the map, you'll notice that white color means background.
2. Function is_on_mask is broken because it ignores 3 channels of RGB picture. (Python complains about indexing). The fix would be to check if all channels are white or non-white, like:
   mask = np.all(this_mask[py[on_mask], px[on_mask]], axis=-1)

[artyompal]

Repro:
mask_map.is_on_mask(10000, 10000)

Or:
mask_map.is_on_mask(10000, 10000, dilation=1) (this results in another error).