Review of #5761: the scale came from zoom × device pixels, without the
96/72 the viewer draws at, and was capped at 8, so a snapshot zoomed in
held fewer pixels than the screen showed: 417 against 534 for 100 pt at
400%. It now takes the viewer's own render scale, and the tile limit is
what stops it. The roadmap entry says when 300 ppi can't be reached.