Update SDL2 to 2.0.3
SDL2: Build fixes, disable dynapi
SDL2 mixer: Backport "Skip JUNK chunks in wave files" b9ed366f0154
SDL2 mixer: Backport "Fixed bug 2690 - Floating point exception in Mix_Volume()" (9599bb0ff844)
SDL2: Backport "Fix audio resampling in some cases." (5da12bedaace)
Update SDL2 path