Fixes a wrong dpi resolution in images exported from composer

In order to avoid wrong dpi resolution in images exported from composer we need to round the dpi resolution calculated "using average of derived x/y dpi" instead of truncate it to int.

src/core/composer/qgscomposition.cpp

@@ -3058,8 +3058,8 @@ QImage QgsComposition::printPageAsRaster( int page, QSize imageSize, int dpi )
   {
     //output size in pixels specified, calculate resolution using average of
     //derived x/y dpi
-    resolution = ( imageSize.width() / mPageWidth
-                   + imageSize.height() / mPageHeight ) / 2.0 * 25.4;
+    resolution = qRound(( imageSize.width() / mPageWidth
+                          + imageSize.height() / mPageHeight ) / 2.0 * 25.4 );
   }
   else if ( dpi > 0 )
   {
@@ -3092,8 +3092,8 @@ QImage QgsComposition::renderRectAsRaster( const QRectF& rect, QSize imageSize,
   {
     //output size in pixels specified, calculate resolution using average of
     //derived x/y dpi
-    resolution = ( imageSize.width() / rect.width()
-                   + imageSize.height() / rect.height() ) / 2.0 * 25.4;
+    resolution = qRound(( imageSize.width() / rect.width()
+                          + imageSize.height() / rect.height() ) / 2.0 * 25.4 );
   }
   else if ( dpi > 0 )
   {