Added Center button to backdrop controls in node editor's side panel

Inlined backdrop's zoom ui limits and hard limits so that
going to minimun and going back to 100 works

release/scripts/ui/space_node.py

@@ -188,6 +188,7 @@ class NODE_PT_properties(bpy.types.Panel):
 
         col = layout.column(align=True)
         col.label(text="Offset:")
+        col.operator("node.view_all", text="Center")
         col.prop(snode, "backdrop_x", text="X")
         col.prop(snode, "backdrop_y", text="Y")
         col.operator("node.backimage_move", text="Move")