Reorder rgb_t's fields and simplify the assembly loop.

Reorder RGB to GRB inside the rgb_t struct to match the order the
hardware accepts.  Since many users of the library will reference
the fields by name anyway, it is relatively transparent and allows
us to simplify away 2/3rds of the assembly.
Support two simultenous outputs of two datasets.

Allows streaming two different sets of LED data to two distinct WS2811
strips connected to pins on two different Atmega ports, in parallel.
3 and 4 simultanous outputs support if pins share the port.