Fix TextureSlotControl not accepting Render Textures (#1569)

* Allow RT input to TextureSlotControl

* Update changelog

com.unity.shadergraph/CHANGELOG.md

 - Sub graphs are now suffixed with (sub), so you can tell them apart from other nodes.
 - Boolean and Texture type properties now function correctly in sub-graphs.
 - The preview of a node does not obstruct the selection outliner anymore.
+- The Texture type default input now accepts render textures.
 - The Dielectric Specular node no longer resets its control values.
 - HD PBR subshader no longer duplicates surface description code into vertex shader.
 - You can now copy, paste, and duplicate sub-graph nodes with vector type input ports.

com.unity.shadergraph/Editor/Drawing/Views/Slots/TextureSlotControlView.cs

 
         void OnValueChanged(ChangeEvent<Object> evt)
         {
-            var texture = evt.newValue as Texture2D;
+            var texture = evt.newValue as Texture;
             if (texture != m_Slot.texture)
             {
                 m_Slot.owner.owner.owner.RegisterCompleteObjectUndo("Change Texture");