The Header and Footer, Watermark and Background dialogs drew their
preview on a blank sheet, so you could not see where the marks would
land on the page's text and pictures. The preview now paints the page's
own thumbnail, rendered off the UI thread like the pages panel's, with a
blank sheet until it arrives. It shows the current page when the page
range selects it, otherwise the first page the range selects, at its
displayed size (after /Rotate), which is the frame the engine lays the
marks out in. A background colour tints the page picture instead of
covering it, as a background behind the content looks.
Fixes#2837.
Guillem Hermida
27a2ac47cf