Update SDL2 to 2.0.9

Just a straight copypasta of the SDL2 dev files into libs. MI would just like to make sure that nothing breaks in-game.

At a surface-level, things look OK to me. It builds and plays successfully.

Download binary: https://www.libsdl.org/release/SDL2-2.0.9-win32-x86.zip

This update is required if Mixer X and non-native MIDI should make it into 2.2.