Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

scene/resources/shape_3d.h

@@ -41,7 +41,7 @@ class Shape3D : public Resource {
 	OBJ_SAVE_TYPE(Shape3D);
 	RES_BASE_EXTENSION("shape");
 	RID shape;
-	real_t margin;
+	real_t margin = 0.04;
 
 	Ref<ArrayMesh> debug_mesh_cache;