Fixes the pointer event point calculation relative to the stage

@eric79 @youngca 
This correctly sets up the stage point and yet another page-stage relative point.  This does fix the drawing after the stage has been dragged on all clients.

Unit placement is still an issue after dragging the map, we need to know not only the stage coordinates for the client who created the unit, but the stage coordinates for the client who is replicating the unit creation.
What is the index on `eventData` for?