Fix positioning of text decoration with some fonts

Actually it just reverts 4db31cbd. When calculating a position for a decoration, we should just apply the decoration's offset to the line's baseline (line.y + line.ascent). The regression was introduced by 54b5287adf4f5b004fcf47840c7f2e1e561a90c1 in Qt 5.6, when we switched from prepending leading to the baseline of text and started appending it instead.

Pick-to: 5.15 6.2 6.4 6.5 Fixes: QTBUG-96700 Fixes: QTBUG-97594 Change-Id: I7f816b71859ffcb6b1c641f0c8b8e1d810bfc525 Reviewed-by: Eskil Abrahamsen Blomfeldt eskil.abrahamsen-blomfeldt@qt.io (cherry picked from commit 5d082dc9)