In GimpProjection, change gimp_projection_add_update_area() to take
coordinates in the projection's coordinate system, rather than the
image coordinate system, and move the offset adjustment to the
projectable invalidation handler.

Modify gimp_projection_projectable_structure_changed() to pass
projection-space coordinates to gimp_projection_add_update_area().

gimp_projection_get_buffer() and
gimp_projection_projectable_bounds_changed() already pass
projection-space coordinates to gimp_projection_add_update_area(),
which was wrong before, when the projection had a nontrivial
offset, but is correct now.

(cherry picked from commit 2d63bc6e)