Driving 8 WS2811 strips in parallel with an 8Mhz AVR

During a very short brainstorming session with Vinnie (where Vinnies part of the conversation consisted of mentioning that he had expected an 8-channel parallel version of the WS2811 driver) it became apparent that it should be possible to drive 8 WS2811 led strings in parallel from one AVR. This allows a single 8Mhz AVR to output at a speed of 266666 RGB LED values per second, or 10000 LEDs at a framerate of 25/s.

Doing this requires that the LED data is transposed in memory, i.e. that the first byte contains the first 8 bits of the channels, the second byte the second 8 bits, etc. I'm currently working on a way to efficiently transpose the bytes in-place but there are many applications for which it is fine to have the data pre-transposed in memory ("bitmap-like" applications can have their bitmaps transposed).

We expect to use this technique in a number of POV applications (spinning globe, anyone?)

The code is below. This is definitely simpler than the single-channel version, but the single channel version had the advantage that the RGB (or rather GRB) values could stay in memory as such, without transposing.