fix shadow issue with lights in mix mode, i think fixes #1611

demos/2d/light_mask/engine.cfg

@@ -2,3 +2,7 @@
 
 name="Using Lights As Mask"
 main_scene="res://lightmask.scn"
+
+[rasterizer]
+
+shadow_filter=3

drivers/gles2/shaders/canvas.glsl

@@ -369,7 +369,7 @@ LIGHT_SHADER_CODE
 #if defined(USE_LIGHT_SHADOW_COLOR)
 	color=mix(shadow_color,color,shadow_attenuation);
 #else
-	color.rgb*=shadow_attenuation;
+	color*=shadow_attenuation;
 #endif
 //use shadows
 #endif