Native SDL2 subset for pydisplay's desktop backend

# Linux / unix port
sudo apt install libsdl2-dev
git clone https://github.com/PyDevices/usdl2.git

# Windows (MinGW) -- point at the official SDL2 dev zip
export SDL2_DEV=~/SDL2-2.30.10

# CPython / Android: TestPyPI usdl2, where applicable