Convert writer_png to C++; add Writer class; add PNGWriter class

src/writer.h

@@ -0,0 +1,21 @@
+/* writer.h
+ *
+ * Writers handle the interface between (mostly) C libraries to write various image formats and the rest of Charles.
+ *
+ * Eryn Wells <eryn@erynwells.me>
+ */
+
+#ifndef __WRITER_H__
+#define __WRITER_H__
+
+#include <string>
+#include "scene.h"
+
+
+class Writer
+{
+public:
+    virtual int write_scene(const Scene &scene, const std::string &filename) = 0;
+};
+
+#endif