CYD white background fix

[DynaMight1124]

Hi

Firstly, thanks for the amazing project. I have recently built a small arcade with this code and its amazing!

I also have a CYD2USB with a ST7789 display, however when I flashed I had a white background rather than black. I know from other projects I've used on this device that it can do that with this display.

I have managed to correct the issue, I'm hoping my below fixes can be added and hoping I dont need to submit a PR because I have no idea how to do that :)

So anyway my fixes involve both config.h and video.cpp.

In config.h I added this line to line 80:
// #define TFT_INV_OFF // some CYD screens need inversion off if you have a white background rather than black
Normal behaviour should probably be commented so it doesn't affect other ST7789 users.

In video.cpp I deleted line 104 and added:
#ifdef TFT_INV_OFF
0x20, 0, // INV OFF
#else
0x21, 0, // INV ON
#endif

I tested the line in config.h commented out and in and it had the correct effects on the display indicating the code is working e.g. white and black backgrounds.

Thanks