Since introducing gimp_display_shell_canvas_tick() we were setting
shell->disp_width and shell->disp_height in the tick callback, which
was too late and caused redundant rendering and scrolling, but was
never noticed.

Now we clear the render cache and its valid region in
gimp_display_shell_canvas_size_allocate() directly and set
shell->disp_width and shell->disp_height immediately, so other places
that listen to the canvas' size-allocate get the right values.

The old size of the canvas gets to the tick callback using a small
struct as user data.