Added material_overlay property to MeshInstance

Applying overlay materials into multi-surface meshes currently
requires adding a next pass material to all the surfaces, which
might be cumbersome when the material is to be applied to a range
of different geometries. This also makes it not trivial to use
AnimationPlayer to control the material in case of visual effects.
The material_override property is not an option as it works
replacing the active material for the surfaces, not adding a new pass.

This commit adds the material_overlay property to GeometryInstance
(and therefore MeshInstance), having the same reach as
material_override (that is, all surfaces) but adding a new material
pass on top of the active materials, instead of replacing them.

Implemented in rasterizer of both GLES2 and GLES3.

servers/visual/rasterizer.h

@@ -88,6 +88,7 @@ public:
 
 		RID skeleton;
 		RID material_override;
+		RID material_overlay;
 
 		Transform transform;