Update shading_language.rst

Fix CanvasItem lighting shader table to make it visible again
Add missing variables to it

reference/shading_language.rst

@@ -447,7 +447,11 @@ CanvasItem (2D) - LightShader
 +-------------------------------------+-------------------------------------------------------------------------------+
 | const color *LIGHT\_COLOR*          | Color of Light                                                                |
 +-------------------------------------+-------------------------------------------------------------------------------+
-| out vec4 *LIGHT*                    | Light Output (shader is ignored if this is not used)                           |
+| vec2 *LIGHT\_UV*                    | UV for light image                                                            |
++-------------------------------------+-------------------------------------------------------------------------------+
+| color *SHADOW*                      | Light shadow color override                                                   |
++-------------------------------------+-------------------------------------------------------------------------------+
+| out vec4 *LIGHT*                    | Light Output (shader is ignored if this is not used)                          |
 +-------------------------------------+-------------------------------------------------------------------------------+
 
 Examples