Add a branch rgb_handles_to_mask which checks to see if the input is already uint8 before multiplying.

[beneisner]

Resolves #204 . There are two callsites for rgb_handles_to_mask, one which reads directly from the sensor (and returns an array on the range [0, 1]) and one which reads from stored png files (and returns an array on the range [0, 255]). This is a bit of a hack, but it's a minimal change which produces the expected behavior when reading from stored demos and when reading from an environment directly.