The only remaining sync call from client to server is now the call
that switches a window's backing store. That one actually relies on
the synchronization to hand over ownership of the backing stores,
so it has to stay synchronous for now.
This patch moves the magnifier rect computation over to the server side
to ensure that the mouse cursor position and the screen image never get
out of sync.
Instead of having the window non-resizable, it now defaults to a frame
size of 200 x 200 pixels but is user-resizable. The shown screenshot
updates its size accordingly. :^)
Sometimes we might need a larger view, or a non-square shape.
