Isolate all PNG loading in a separate source/header.

This is in preparation of replacing SDL_image with direct use of libpng.

src/surfacecollection.cpp

@@ -23,6 +23,8 @@
 #include "utilities.h"
 #include "debug.h"
 
+#include <iostream>
+
 using std::endl;
 using std::string;