Properly reserve place for arrays in line and polygon clipping routines

A QVector::clear() method completely reinitializes the object. Calling
a QVector::reserve() before clear() is useless and causes big overhead at
array append operations.

Give ~6% perfomance gain.

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>

Author: jekhor

src/core/qgsclipper.cpp

@@ -49,8 +49,8 @@ const unsigned char* QgsClipper::clippedLineWKB( const unsigned char* wkb, const
   double p1x_c, p1y_c; //clipped end coordinates
   double lastClipX = 0.0, lastClipY = 0.0; //last successfully clipped coords
 
-  line.reserve( nPoints + 1 );
   line.clear();
+  line.reserve( nPoints + 1 );
 
   for ( unsigned int i = 0; i < nPoints; ++i )
   {

src/core/qgsclipper.h

@@ -179,11 +179,11 @@ inline void QgsClipper::trimPolygon( QPolygonF& pts, const QgsRectangle& clipRec
   tmpPts.reserve( pts.size() );
 
   trimPolygonToBoundary( pts, tmpPts, clipRect, XMax, clipRect.xMaximum() );
-  pts.clear();
+  pts.resize( 0 );
   trimPolygonToBoundary( tmpPts, pts, clipRect, YMax, clipRect.yMaximum() );
-  tmpPts.clear();
+  tmpPts.resize( 0 );
   trimPolygonToBoundary( pts, tmpPts, clipRect, XMin, clipRect.xMinimum() );
-  pts.clear();
+  pts.resize( 0 );
   trimPolygonToBoundary( tmpPts, pts, clipRect, YMin, clipRect.yMinimum() );
 }