Light2D shadow mask construction fix

When using the default setting (layer 1 set only) nothing is stored in the tscn file for a Light2D, hence it relies on the value in the constructor.

The problem is the constructed value is 1 in Light2D, and -1 in RasterizerCanvas::Light. -1 results in all bits being set so all occluders are shown, rather than just those in layer 1.

This PR changes Rasterizer::Canvas constructor to set to 1. An alternative is to have -1 as the value for layer 1 throughout.

servers/visual/rasterizer.h

@@ -665,7 +665,7 @@ public:
 			item_mask = 1;
 			scale = 1.0;
 			energy = 1.0;
-			item_shadow_mask = -1;
+			item_shadow_mask = 1;
 			mode = VS::CANVAS_LIGHT_MODE_ADD;
 			texture_cache = NULL;
 			next_ptr = NULL;