Update SDL2 to newest version to fix 7.1 playback

This fixes 7.1 playback. Before this commit, SDL used 7.1 front audio configuration with Center Left and Center Right speakers, but without Rear speakers.

See https://github.com/libsdl-org/SDL/issues/11709 for more details.

libs/SDL2/include/SDL_revision.h.cmake

@@ -0,0 +1,8 @@
+#cmakedefine SDL_VENDOR_INFO "@SDL_VENDOR_INFO@"
+#define SDL_REVISION_NUMBER 0
+
+#ifdef SDL_VENDOR_INFO
+#define SDL_REVISION "@SDL_REVISION@ (" SDL_VENDOR_INFO ")"
+#else
+#define SDL_REVISION "@SDL_REVISION@"
+#endif