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.
If we're sending out a configure event, we can't immediately move the
window; we need to instead wait to apply the new position when the
client sends a new buffer.
dx/dy should be against the regular window's rect, and need to
be ignored when we're resizing. Instead, we use gravity to anchor
the window's new rectangle when resizing.
