Update Mac SDL library version to 2.0.9 #405
This is meant as a temporary fix for the scroll wheel issue (#379). This updates the Mac SDL dylib, but does not include header update or other operating system library updates so there is no risk of regressions for other operating systems.
I copied the SDL dylib from iortcw, at https://github.com/iortcw/iortcw/blob/master/MP/code/libs/macosx/libSDL2-2.0.0.dylib, since I'm not sure how to compile SDL as a dylib myself.
Based on a quick test, I was able to compile and run the game under High Sierra with this patch and it no longer exhibits the scroll wheel issue.