T4 Support

I updated the comments in readme plus example apps, showing pin numbers.

I also fixed pin 17.  So far I have only tried 1, 8, 14, 16, 20 as simply using breadboard version

It is building now and I am getting outputs.

However, I had to fix IMXRT.h in core\teensy4 as the
DMAMUX_SOURCE_LPUARTx_TX and ....RX were reversed in numbers
DMAMUX_SOURCE_LPUART6_TX was 71 and supposed to be 70

Add function to clear all pixels