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/projects/owatcom/libpng.wpj

@@ -0,0 +1,92 @@
+40
+projectIdent
+0
+VpeMain
+1
+WRect
+256
+0
+8960
+9284
+2
+MProject
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+MCommand
+322
+# Locations of zlib and (if required) awk (change as required:)
+set zlib=..\..\..\zlib
+set awk=
+#
+@if not exist pngconfig.dfa $(MAKE) $(__MAKEOPTS__) -f pngconfig.mak defaults
+@if exist config.inf type config.inf
+@echo Checking for the libpng configuration file pnglibconf.h
+$(MAKE) $(__MAKEOPTS__) -f pngconfig.mak
+4
+MCommand
+19
+@type pngconfig.inf
+3
+5
+WFileName
+10
+libpng.tgt
+6
+WFileName
+11
+pngtest.tgt
+7
+WFileName
+12
+pngvalid.tgt
+8
+WVList
+3
+9
+VComponent
+10
+WRect
+0
+0
+5632
+4164
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+0
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+WFileName
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+libpng.tgt
+0
+0
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+VComponent
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+WRect
+1280
+1540
+5632
+4164
+0
+0
+14
+WFileName
+11
+pngtest.tgt
+0
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+VComponent
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+WRect
+518
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+0
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+WFileName
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+pngvalid.tgt
+0
+1
+9