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/test-pngtest.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+echo "Running tests.  For details see pngtest-log.txt"
+
+echo "============ pngtest pngtest.png ==============" > pngtest-log.txt
+
+echo "Running test-pngtest.sh"
+if ./pngtest --strict ${srcdir}/pngtest.png >> pngtest-log.txt 2>&1
+then
+  echo "  PASS: pngtest --strict pngtest.png"
+  err=0
+else
+  echo "  FAIL: pngtest --strict pngtest.png"
+  err=1
+fi
+exit $err