Hopefully fix #10599 (blank composer map when rendering rasters)

When printing on Windows, the printing does not seem to work
well in the worker thread as QImages get converted to QPixmaps.
Therefore we force the map rendering into main thread to avoid the issues.

I do not have a printer, so I can't confirm the fix really helps

python/core/qgsmaprendererjob.sip

@@ -218,6 +218,17 @@ class QgsMapRendererCustomPainterJob : QgsMapRendererJob
      */
     void waitForFinishedWithEventLoop( QEventLoop::ProcessEventsFlags flags = QEventLoop::AllEvents );
 
+    /**
+     * Render the map synchronously in this thread. The function does not return until the map
+     * is completely rendered.
+     *
+     * This is an alternative to ordinary API (using start() + waiting for finished() signal).
+     * Users are discouraged to use this method unless they have a strong reason for doing it.
+     * The synchronous rendering blocks the main thread, making the application unresponsive.
+     * Also, it is not possible to cancel rendering while it is in progress.
+     */
+    void renderSynchronously();
+
   protected slots:
     void futureFinished();
 

src/core/composer/qgscomposermap.cpp

@@ -221,9 +221,10 @@ void QgsComposerMap::draw( QPainter *painter, const QgsRectangle& extent, const
 
   // render
   QgsMapRendererCustomPainterJob job( jobMapSettings, painter );
-  job.start();
-  // wait, but allow network requests to be processed
-  job.waitForFinishedWithEventLoop( QEventLoop::ExcludeUserInputEvents );
+  // Render the map in this thread. This is done because of problems
+  // with printing to printer on Windows (printing to PDF is fine though).
+  // Raster images were not displayed - see #10599
+  job.renderSynchronously();
 }
 
 void QgsComposerMap::cache( void )

src/core/qgsmaprendererjob.cpp

@@ -182,6 +182,7 @@ QgsMapRendererCustomPainterJob::QgsMapRendererCustomPainterJob( const QgsMapSett
     , mPainter( painter )
     , mLabelingEngine( 0 )
     , mActive( false )
+    , mRenderSynchronously( false )
 {
   QgsDebugMsg( "QPAINTER construct" );
 }
@@ -237,6 +238,13 @@ void QgsMapRendererCustomPainterJob::start()
 
   QgsDebugMsg( "Rendering prepared in (seconds): " + QString( "%1" ).arg( prepareTime.elapsed() / 1000.0 ) );
 
+  if ( mRenderSynchronously )
+  {
+    // do the rendering right now!
+    doRender();
+    return;
+  }
+
   // now we are ready to start rendering!
   connect( &mFutureWatcher, SIGNAL( finished() ), SLOT( futureFinished() ) );
 
@@ -311,6 +319,15 @@ void QgsMapRendererCustomPainterJob::waitForFinishedWithEventLoop( QEventLoop::P
 }
 
 
+void QgsMapRendererCustomPainterJob::renderSynchronously()
+{
+  mRenderSynchronously = true;
+  start();
+  futureFinished();
+  mRenderSynchronously = false;
+}
+
+
 void QgsMapRendererCustomPainterJob::futureFinished()
 {
   mActive = false;

src/core/qgsmaprendererjob.h

@@ -278,6 +278,17 @@ class CORE_EXPORT QgsMapRendererCustomPainterJob : public QgsMapRendererJob
      */
     void waitForFinishedWithEventLoop( QEventLoop::ProcessEventsFlags flags = QEventLoop::AllEvents );
 
+    /**
+     * Render the map synchronously in this thread. The function does not return until the map
+     * is completely rendered.
+     *
+     * This is an alternative to ordinary API (using start() + waiting for finished() signal).
+     * Users are discouraged to use this method unless they have a strong reason for doing it.
+     * The synchronous rendering blocks the main thread, making the application unresponsive.
+     * Also, it is not possible to cancel rendering while it is in progress.
+     */
+    void renderSynchronously();
+
   protected slots:
     void futureFinished();
 
@@ -296,6 +307,7 @@ class CORE_EXPORT QgsMapRendererCustomPainterJob : public QgsMapRendererJob
 
     bool mActive;
     LayerRenderJobs mLayerJobs;
+    bool mRenderSynchronously;
 };