Freetype: adjust underlinePosition to match our expectations

We expect underlinePosition() to be the topleft corner of where we
start drawing the underline but freetype reports the _center_ of the
underline:

"underline_position: [...]. It is the center of the underlining stem."
- https://freetype.org/freetype2/docs/reference/ft2-base_interface.html

Fixes: QTBUG-62602
Change-Id: I96808816f50f3f37b18f579adf285502c88e6ea5
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Ievgenii Meshcheriakov <ievgenii.meshcheriakov@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
(cherry picked from commit 0062f5a2089f9654c8880087ac6fb0b46a90bef2)
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/platformsupport/fontdatabases/freetype/qfontengine_ft.cpp b/src/platformsupport/fontdatabases/freetype/qfontengine_ft.cpp
index e5de3953ff..6d40dcb22a 100644
--- a/src/platformsupport/fontdatabases/freetype/qfontengine_ft.cpp
+++ b/src/platformsupport/fontdatabases/freetype/qfontengine_ft.cpp
@@ -756,7 +756,8 @@ bool QFontEngineFT::init(FaceId faceId, bool antialias, GlyphFormat format,
         }
         // underline metrics
         line_thickness =  QFixed::fromFixed(FT_MulFix(face->underline_thickness, face->size->metrics.y_scale));
-        underline_position = QFixed::fromFixed(-FT_MulFix(face->underline_position, face->size->metrics.y_scale));
+        QFixed center_position = QFixed::fromFixed(-FT_MulFix(face->underline_position, face->size->metrics.y_scale));
+        underline_position = center_position - line_thickness / 2;
     } else {
         // ad hoc algorithm
         int score = fontDef.weight * fontDef.pixelSize;