check zoom bounds in dt_dev_process_image_job()

Check zoom bounds here every time rather than in zoom_key_accel().
This now catches the case of toggling from 200% to 100% at image edge,
but also a case when switching between aspect ratios or changing image
orientation.

Note that now dt_dev_check_zoom_bounds() gets called more times than
necessary -- either unnecessarily in dt_dev_process_image_job() or
gets checked in other functions as well as in
dt_dev_process_image_job().