Handle raster layer's transparency band while rendering (fix #2491)

Author: brushtyler

src/app/qgsrasterlayerproperties.cpp

@@ -178,6 +178,8 @@ QgsRasterLayerProperties::QgsRasterLayerProperties( QgsMapLayer* lyr, QgsMapCanv
     cboGreen->addItem( myRasterBandName );
     cboBlue->addItem( myRasterBandName );
     cboxColorMapBand->addItem( myRasterBandName );
+    cboxTransparencyBand->addItem( myRasterBandName );
+    cboxTransparencyBand->setEnabled( true );
   }
 
   cboRed->addItem( TRSTRING_NOT_SET );