Core: Add constexpr constructors/operators to math structs

• Begin integrating `constexpr` on math tests; use `static_assert` where appropriate

editor/import/3d/collada.cpp

@@ -208,7 +208,6 @@ Vector<float> Collada::AnimationTrack::get_value_at_time(float p_time) const {
 
 				Vector<float> ret;
 				ret.resize(16);
-				Transform3D tr;
 				// i wonder why collada matrices are transposed, given that's opposed to opengl..
 				ret.write[0] = interp.basis.rows[0][0];
 				ret.write[1] = interp.basis.rows[0][1];

editor/plugins/canvas_item_editor_plugin.cpp

@@ -4872,7 +4872,6 @@ void CanvasItemEditor::_set_owner_for_node_and_children(Node *p_node, Node *p_ow
 }
 
 void CanvasItemEditor::_focus_selection(int p_op) {
-	Vector2 center(0.f, 0.f);
 	Rect2 rect;
 	int count = 0;
 

editor/plugins/gizmos/camera_3d_gizmo_plugin.cpp

@@ -210,7 +210,6 @@ void Camera3DGizmoPlugin::redraw(EditorNode3DGizmo *p_gizmo) {
 
 			Vector3 right, up;
 			Vector3 back(0, 0, -1.0);
-			Vector3 front(0, 0, 0);
 
 			if (aspect == Camera3D::KeepAspect::KEEP_WIDTH) {
 				right = Vector3(keep_size, 0, 0);

editor/plugins/polygon_3d_editor_plugin.cpp

@@ -475,7 +475,7 @@ void Polygon3DEditor::_polygon_draw() {
 		va.resize(poly.size());
 		Vector3 *w = va.ptrw();
 		for (int i = 0; i < poly.size(); i++) {
-			Vector2 p, p2;
+			Vector2 p;
 			p = i == edited_point ? edited_point_pos : poly[i];
 
 			Vector3 point = Vector3(p.x, p.y, depth);