Use bundled libpng by default when building for Windows

This was the default before commit bace05c, and is consistent with
how everything else works in Windows builds. In particular, we use the
bundled zlib by default, but libpng depends on zlib, so we shouldn't
use a system libpng unless we are also using a (compatible)
system zlib.

CMakeLists.txt

@@ -59,6 +59,7 @@ set(UseInternalSDL2Default   OFF)
 if(WIN32)
   set(UseInternalOpenALDefault ON)
   set(UseInternalZlibDefault   ON)
+  set(UseInternalPNGDefault    ON)
   set(UseInternalJPEGDefault   ON)
   set(UseInternalSDL2Default   ON)
 endif()