`ws2812` buffer extracted to new `pixbuf` module.
* The new pixbuf module has more functionality than the `ws2812`-specific buffer it replaces.
* This is work in progress towards https://github.com/nodemcu/nodemcu-firmware/issues/2916
* The LED driver modules `ws2812`, `ws2801`, `apa102`, and `tm1829` have sprouted `pixbuf` support.
* `NTest` tests for `pixbuf` now exist.
While here, document the ws2812 UART-based overlapping with mainline
execution. Fixes https://github.com/nodemcu/nodemcu-firmware/issues/3140
Co-authored-by: Gregor Hartmann <HHHartmann@users.noreply.github.com>
Nathaniel Wesley Filardo