Correctly position aliased lines with flat caps

The code was mispositioning lines by half a pixel, as it
added half a pixel offset and then rounded in addition.
This submit fixes this and also removes certain artifacts when
drawing rects at .5 pixel positions.

Lance now doesn't show any significant differences to the
4.7 rendering anymore.

Task-number: QTBUG-20199
Reviewed-by: Aavit
(cherry picked from commit 49409f612c47f30434aa809e4d2c963f1a6bb88a)

Change-Id: Iab3936e688eba16b82f5cdb4f36f54af807d78ea
Reviewed-on: http://codereview.qt.nokia.com/3260
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/gui/painting/qcosmeticstroker_p.h b/src/gui/painting/qcosmeticstroker_p.h
index d7bd79ad15..53cdf2c0ac 100644
--- a/src/gui/painting/qcosmeticstroker_p.h
+++ b/src/gui/painting/qcosmeticstroker_p.h
@@ -78,10 +78,12 @@ public:
 
     // used to avoid drop outs or duplicated points
     enum Direction {
-        TopToBottom,
-        BottomToTop,
-        LeftToRight,
-        RightToLeft
+        TopToBottom = 0x1,
+        BottomToTop = 0x2,
+        LeftToRight = 0x4,
+        RightToLeft = 0x8,
+        VerticalMask = 0x3,
+        HorizontalMask = 0xc
     };
 
     QCosmeticStroker(QRasterPaintEngineState *s, const QRect &dr)