This moves all RX packet processing into interrupt context, removing the queueing bottleneck that previous firmwares experienced. Before this change, we could handle at most ~3 packets before they would start NAK'ing due to lack of buffer space until the next main loop iteration pulled additional packets into the fbNew buffer. Now packets can immediately go into fbNew unless there's a pending frame finalization. This vastly improves the efficiency of the USB protocol, and we need far fewer NAKs.

This makes it easier to move around for future optimizations, and it already gains us a few FPS by allowing the compiler to optimize a little better

Sometimes we might want host-side software to control it, or even to just turn it off.

This makes the mapping from 16-bit color to LUT entry straightforward, while still fully defining the entire range from 0x0000 through 0xFFFF. This fixes an edge case where colors greater than 0xFF00 would wrap past the end of the R/G LUTs onto the next color channel.