Introducing power throttling

[davepl]

With a pending checkin I've added power throttle for the matrix projects like Mesmerizer. Right now it sets a max target of 5W. Each frame, it runs all the pixels and does a power estimation based on their R, G B components. It then scales back the master brightness in order to keep it under the target.

Brightness is ramped back in gradually via a low pass filter (weighted average) to avoid flashing.