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-pngvalid-full.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+# Run a sequence of gamma tests quietly
+err=0
+
+echo >> pngtest-log.txt
+echo "============ pngvalid-full.sh ==============" >> pngtest-log.txt
+
+echo "Running test-pngvalid-full.sh"
+for gamma in threshold transform sbit 16-to-8 background alpha-mode "transform --expand16" "background --expand16" "alpha-mode --expand16"
+do
+   if ./pngvalid "$@" --gamma-$gamma >> pngtest-log.txt 2>&1
+   then
+      echo "  PASS: pngvalid" "$@" "--gamma-$gamma"
+   else
+      echo "  FAIL: pngvalid" "$@" "--gamma-$gamma"
+      err=1
+   fi
+done
+
+exit $err