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/pngusr.dfa

@@ -0,0 +1,14 @@
+# pngusr.dfa
+#
+# Build time configuration of libpng
+#
+# Enter build configuration options in this file
+#
+# Security settings: by default these limits are unset, you can change them
+# here by entering the appropriate values as #defines preceded by '@' (to cause,
+# them to be passed through to the build of pnglibconf.h), for example:
+#
+# @# define PNG_USER_WIDTH_MAX 1000000
+# @# define PNG_USER_HEIGHT_MAX 1000000
+# @# define PNG_USER_CHUNK_CACHE_MAX 128
+# @# define PNG_USER_CHUNK_MALLOC_MAX 8000000