Issue #10676: Fix preview for Jigsaw GUI

This patch attempts to resolve Issue #10676 (closed).

After the port to gimp_procedure_dialog_get_drawable_preview (), Jigsaw was was switched from using GimpAspectPreview to GimpDrawablePreview. This meant it no longer auto-resized the preview box to match the thumbnail dimensions. So the preview box had scrollbars for the full drawable dimensions, but only the area of the thumbnail was actually drawn - leaving the rest as glitchy colors left over from the original image.

To resolve, gimp_preview_set_bounds () and gimp_preview_set_size () are now called once the preview size is determined to fit the widget to the thumbnail dimensions. This returns the widget to its 2.10 behavior.

@ajonsson, when you have time could you test this out and see if it fixes the problem for you? I think the Blind issue is similar, but I want to make sure this is right before moving on to that.