update libpng to 1.5.10, add precompiled library for ffmpeg 0.10.2 for mac, and Xcode 4.3 project in a separate project.

This should make it possible to build StepMania 5 on Mac OS X.
The catch is that it will only work with Snow Leopard and up, but I think this is better than nothing.

We can support PowerPC / 10.4-10.5 again when someone can make the old Xcode 3 project work.

extern/libpng/include/libpng.pc.in

@@ -1,11 +1,11 @@
-prefix=@prefix@
-exec_prefix=@exec_prefix@
-libdir=@libdir@
-includedir=@includedir@/libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@
-
-Name: libpng
-Description: Loads and saves PNG files
-Version: @PNGLIB_VERSION@
-Libs: -L${libdir} -lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@
-Libs.private: @LIBS@
-Cflags: -I${includedir}
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@/libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@
+
+Name: libpng
+Description: Loads and saves PNG files
+Version: @PNGLIB_VERSION@
+Libs: -L${libdir} -lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@
+Libs.private: @LIBS@
+Cflags: -I${includedir}