Turn antialising render flag on when appropriate in QgsRenderContext:…

…:fromQPainter()

src/core/qgsrendercontext.cpp

@@ -105,6 +105,10 @@ QgsRenderContext QgsRenderContext::fromQPainter( QPainter *painter )
   {
     context.setScaleFactor( 3.465 ); //assume 88 dpi as standard value
   }
+  if ( painter && painter->renderHints() & QPainter::Antialiasing )
+  {
+    context.setFlag( QgsRenderContext::Antialiasing, true );
+  }
   return context;
 }
 

tests/src/python/test_qgsrendercontext.py

@@ -73,15 +73,18 @@ def testFromQPainter(self):
         p = QPainter()
         c = QgsRenderContext.fromQPainter(p)
         self.assertEqual(c.painter(), p)
+        self.assertEqual(c.testFlag(QgsRenderContext.Antialiasing), False)
         self.assertAlmostEqual(c.scaleFactor(), 88 / 25.4, 3)
 
         im = QImage(1000, 600, QImage.Format_RGB32)
         dots_per_m = 300 / 25.4 * 1000  # 300 dpi to dots per m
         im.setDotsPerMeterX(dots_per_m)
         im.setDotsPerMeterY(dots_per_m)
         p = QPainter(im)
+        p.setRenderHint(QPainter.Antialiasing)
         c = QgsRenderContext.fromQPainter(p)
         self.assertEqual(c.painter(), p)
+        self.assertEqual(c.testFlag(QgsRenderContext.Antialiasing), True)
         self.assertAlmostEqual(c.scaleFactor(), dots_per_m / 1000, 3)  # scaleFactor should be pixels/mm
 
     def testFromMapSettings(self):