Fix shaky lines on 32-bit systems

The value of the mirrored x coordinate is almost exactly integer
all the time. On 64-bit systems the CPU converts it into integer
correctly, but on 32-bit systems it probably doesn't download it
from the FPU register (which are 80-bit and therefore more precise),
so the value got truncated to the decremented value. It results in the
lines becoming shaky due to random precision errors.

Now we explicitly round "almost-integer" part of the sum so no precision
fluctuations happen.

BUG:341577

krita/image/brushengine/kis_paintop.cc

@@ -104,15 +104,8 @@ void KisPaintOp::setFanCornersInfo(bool fanCornersEnabled, qreal fanCornersStep)
 
 void KisPaintOp::splitCoordinate(qreal coordinate, qint32 *whole, qreal *fraction)
 {
-    qint32 i = static_cast<qint32>(coordinate);
-
-    if (coordinate < 0) {
-        // We always want the fractional part to be positive.
-        // E.g. -1.25 becomes -2 and +0.75
-        i--;
-    }
-
-    qreal f = coordinate - i;
+    const qint32 i = std::floor(coordinate);
+    const qreal f = coordinate - i;
 
     *whole = i;
     *fraction = f;

krita/plugins/paintops/libpaintop/kis_dab_cache.cpp

@@ -252,8 +252,8 @@ KisFixedPaintDeviceSP KisDabCache::tryFetchFromCache(const SavedDabParameters &p
 }
 
 qreal positiveFraction(qreal x) {
-    int unused;
-    qreal fraction;
+    qint32 unused = 0;
+    qreal fraction = 0.0;
     KisPaintOp::splitCoordinate(x, &unused, &fraction);
 
     return fraction;
@@ -267,8 +267,8 @@ KisDabCache::calculateDabRect(const QPointF &cursorPoint,
                               const KisPaintInformation& info,
                               const MirrorProperties &mirrorProperties)
 {
-    int x, y;
-    qreal subPixelX, subPixelY;
+    qint32 x = 0, y = 0;
+    qreal subPixelX = 0.0, subPixelY = 0.0;
 
     if (mirrorProperties.coordinateSystemFlipped) {
         angle = 2 * M_PI - angle;
@@ -296,13 +296,13 @@ KisDabCache::calculateDabRect(const QPointF &cursorPoint,
     if (mirrorProperties.horizontalMirror) {
         subPixelX = positiveFraction(-(cursorPoint.x() + hotSpot.x()));
         width = m_d->brush->maskWidth(scaleX, angle, subPixelX, subPixelY, info);
-        x = cursorPoint.x() + subPixelX + hotSpot.x() - width;
+        x = qRound(cursorPoint.x() + subPixelX + hotSpot.x()) - width;
     }
 
     if (mirrorProperties.verticalMirror) {
         subPixelY = positiveFraction(-(cursorPoint.y() + hotSpot.y()));
         height = m_d->brush->maskHeight(scaleY, angle, subPixelX, subPixelY, info);
-        y = cursorPoint.y() + subPixelY + hotSpot.y() - height;
+        y = qRound(cursorPoint.y() + subPixelY + hotSpot.y()) - height;
     }
 
     return DabPosition(QRect(x, y, width, height),