support for 2D shadow casters

Added support for 2D shadow casters.

*DANGER* Shaders in CanvasItem CHANGED, if you are using shader in a
CanvasItem and pull this, you will lose them. Shaders now work through a
2D material system similar to 3D. If you don't want to lose the 2D
shader code, save the shader as a .shd, then create a material in
CanvasItem and re-assign the shader.

scene/scene_string_names.cpp

@@ -41,6 +41,7 @@ SceneStringNames::SceneStringNames() {
 	visibility_changed=StaticCString::create("visibility_changed");
 	input_event=StaticCString::create("input_event");
 	shader_shader=StaticCString::create("shader/shader");
+	shader_unshaded=StaticCString::create("shader/unshaded");
 	enter_tree=StaticCString::create("enter_tree");
 	exit_tree=StaticCString::create("exit_tree");
 	item_rect_changed=StaticCString::create("item_rect_changed");