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.

core/math/face3.cpp

@@ -29,7 +29,8 @@
 /*************************************************************************/
 
 #include "face3.h"
-#include "geometry.h"
+
+#include "core/math/geometry.h"
 
 int Face3::split_by_plane(const Plane &p_plane, Face3 p_res[3], bool p_is_point_over[3]) const {