...outside area of Crop Tool -> Highlight option
Add "highlight-opacity" property and turn the controlling GUI into an
expanding toggle that reveals an opacity slider.
Pass the display scroll offset down to gimp_cairo_stipple_pattern_create()
and set it as offset on the created cairo pattern, so stipple patterns
look the same no matter how the display is scrolled.
With gimp_guide_custom_new(), you can create a custom guide with a different
style on canvas (other pattern/color/width). A custom guide won't be saved
and could be used, for instance, for specific GEGL op guiding.
