Support num_strands=1

[jepler]

Over in #4, @gamblor21 identified that the 1 strand case is still not supported even with that PR. This is too bad, because the background write feature of NeoPxl8 increases framerate measurably.

To reduce memory usage and processing time I propose to add a third PIO program supporting a single strand in the original in-memory format. Still take a copy of the data for background writing without "tearing".