For the server-initiated resize case, like unmaximize or some forms
of tiling, we dropped the x/y of the server-assigned rectangle on the
floor, which meant the surface didn't move to where it needed to be in
that case. Now, save it internally, and combine it with the dx/dy passed
in during attaches to figure out where we actually need to be.
This fixes incorrect surface placement after unmaximization.
e6b0525c70