This is a child widget to the MksDisplay which contains just the paintable
region of the remote screen. Doing so allows us to avoid having to handle
crossing events carefully for things like cursors.
Instead, now the MksDisplay calculates the area for the picture and
allocates it there. The MksDisplayPicture will always snapshot the
paintable using the full area it has been allocated.
Event processing can also largely move into the Picture widget which
simplifies bounds checking.
