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/vector3.h

@@ -31,10 +31,10 @@
 #ifndef VECTOR3_H
 #define VECTOR3_H
 
-#include "math_defs.h"
-#include "math_funcs.h"
-#include "typedefs.h"
-#include "ustring.h"
+#include "core/math/math_defs.h"
+#include "core/math/math_funcs.h"
+#include "core/typedefs.h"
+#include "core/ustring.h"
 
 class Basis;
 
@@ -150,13 +150,8 @@ struct Vector3 {
 	}
 };
 
-#ifdef VECTOR3_IMPL_OVERRIDE
-
-#include "vector3_inline.h"
-
-#else
-
-#include "matrix3.h"
+// Should be included after class definition, otherwise we get circular refs
+#include "core/math/matrix3.h"
 
 Vector3 Vector3::cross(const Vector3 &p_b) const {
 
@@ -451,6 +446,4 @@ Vector3 Vector3::reflect(const Vector3 &p_normal) const {
 	return 2.0 * p_normal * this->dot(p_normal) - *this;
 }
 
-#endif
-
 #endif // VECTOR3_H