Instead just transform the measurement extremities appropriately to
still map to the same points.
To do so, I also added out parameters to gimp_image_resize_to_layers()
so that calling code can get offsets from old origin (as well as new
image dimensions).
Add a GimpFillType argument to GimpItem::resize() and fill type
widgets to the canvas and layer resize dialogs. Fill the new parts of
the drawable according to fill type in gimp_drawable_resize(). Make
sure places that need the old behavior get GIMP_FILL_TRANSPARENT
passed by hardcoding it in the GimpItem::resize() implemetations of
channel, mask, selection etc.
