Move File-opening of PNG image into Png constructor (#2161)

Also rename `File::c_str` to `display_name` for clarity

include/gfx/png.hpp

@@ -5,6 +5,7 @@
 
 #include <stdint.h>
 #include <streambuf>
+#include <string>
 #include <vector>
 
 #include "gfx/rgba.hpp"
@@ -17,7 +18,11 @@ struct Png {
 	bool isIndexed = false;
 
 	Png() {}
+	Png(std::string const &path);
 	Png(char const *filename, std::streambuf &file);
+
+private:
+	void initialize(char const *filename, std::streambuf &file);
 };
 
 #endif // RGBDS_GFX_PNG_HPP