Implements the paper 'A Closed Form Solution to Natural Image Matting'
by Levin et al. A user specifies image regions as: foreground, background,
and unknown (in our implementation: white, black, transparent).

The implementation is based on the MATLAB implementation published by
Levin et al.

Care must be taken on large images, due to the algorithm's memory
requirements; specifying smaller unknown regions will reduce memory
consumption.

Execution speed may be increased by increasing 'levels', and
'active_levels'. These specify the depth of downsampled solutions, and
the number of these which are directly solved solutions.