which means we can't setup scrollbars there. Move the code to a
GtkTickCallback which runs before the next frame after the
size-allocate.

Also put the center_image_on_size_allocate() code there because it has
to run after the canvas' tick callback, and the order of tick
callbacks can't be controlled.

As a side effect we now have a flag in GimpDisplayShell which
indicates that there will be a size allocate before the next frame, so
simply skip drawing the canvas completely. This fixes new images
jumping around when they are first shown.