This is an adaptation of @vha3's Realtime Audio FFT to VGA Display with RP2040.
It has been partially rewritten to favour C++isms and support a 240x240 SPI LCD.
See the original code in action: https://www.youtube.com/watch?v=8aibPy4yzCk and a detailed explanation of the FFT code.
I have taken many liberties with this code, possibly breaking it in subtle ways. I recommend referring to the original!