Add gimp_display_shell_[un]transform_with_scale(), which are
similar to gimp_display_shell_[un]transform(), however, they
transform the bounding box to/from uniformly-scaled image space,
given the scale factor as a parameter.  These functions are more
accurate than using gimp_display_shell_[un]transform() and applying
the scaling separately, in particular, when the scale matches the
(horizontal or vertical) display scale.

Use these functions in gimp_display_shell_draw_image(), to avoid an
off-by-one error when transforming screen-space chunks to image-
space chunks, which leads to the symptoms described in the bug.

Fix another potential off-by-one error affecting non-uniformly
scaled images, and window scale factors other than 1.