cairo: Correctly render table during scrolling.

Commit 845d4b4f3f (cairo: Draw table titles in xr_rendering_draw()
too) started rendering table titles in the GUI but forgot to
compensate for this in the call to render_page_draw_region(), so
scrolling caused visible damage.

Bug #31569.
Reported-by: John Darrington <john@darrington.wattle.id.au>