Fill image with 0 in rendering thread (#3722)

Calling QImage::fill( 0 ) on rendering start takes about 40% of the
time required for setting up a new job.
Since this is done on the main thread, the interface feels more
snappy the quicker the setup process is completed.

Author: m-kuhn

src/core/qgsmaprenderercustompainterjob.cpp

@@ -244,6 +244,9 @@ void QgsMapRendererCustomPainterJob::doRender()
       QTime layerTime;
       layerTime.start();
 
+      if ( job.img )
+        job.img->fill( 0 );
+
       job.renderer->render();
 
       job.renderingTime = layerTime.elapsed();

src/core/qgsmaprendererjob.cpp

@@ -286,7 +286,6 @@ LayerRenderJobs QgsMapRendererJob::prepareJobs( QPainter* painter, QgsLabelingEn
         layerJobs.removeLast();
         continue;
       }
-      mypFlattenedImage->fill( 0 );
 
       job.img = mypFlattenedImage;
       QPainter* mypPainter = new QPainter( job.img );

src/core/qgsmaprendererparalleljob.cpp

@@ -215,6 +215,9 @@ void QgsMapRendererParallelJob::renderLayerStatic( LayerRenderJob& job )
   if ( job.cached )
     return;
 
+  if ( job.img )
+    job.img->fill( 0 );
+
   QTime t;
   t.start();
   QgsDebugMsgLevel( QString( "job %1 start (layer %2)" ).arg( reinterpret_cast< ulong >( &job ), 0, 16 ).arg( job.layerId ), 2 );