Improved panning of zoomed images:

- Panning in images is faster now
- When panning, the mouse pointer becomes invisible. After panning
  it reappears at the position where panning started.
- Don't switch to panning mode when whole image is visible. Instead
  the image could be dragged and dropped somewhere (not implemented
  yet).

See review request for remaining issues.

REVIEW: 107985
CCBUG: 293340