Revert SDL 2.0.9 and Mixer 2.0.4

This changes back the SDL compile libraries to 2.0.7, like they were in 2.1.20.

Doing this because the switch to 2.0.9 DLLs didn't work so well due to the sound speedup glitch happening to a few users. It should've been fixed for that version, but evidently not so.

So this just changes things back to the way they were.

(FYI Kart: I ninja-edited the vanilla installers to use 2.1.20's DLLs, so maybe you can continue to bundle those same DLLs for your next release. Sorry for the confusion.)