Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

drivers/png/resource_saver_png.h

@@ -31,8 +31,8 @@
 #ifndef RESOURCE_SAVER_PNG_H
 #define RESOURCE_SAVER_PNG_H
 
-#include "image.h"
-#include "io/resource_saver.h"
+#include "core/image.h"
+#include "core/io/resource_saver.h"
 
 class ResourceSaverPNG : public ResourceFormatSaver {
 public: