[layouts] Fix first background map render can never be canceled

and must always complete

This fixes an issue where map items appear to get "stuck" showing the
"Rendering map" text while the very first render of the map
item completes. If the map render is lengthy (e.g. due to
complex labeling and an initially too small map scale), then
it can appear as if the map item has got stuck.

Instead, we now correctly support cancelation of this initial
map render job, just like we do subsequent ones.

(cherry picked from commit 591bb75)

src/core/layout/qgslayoutitemmap.cpp

@@ -778,7 +778,13 @@ void QgsLayoutItemMap::paint( QPainter *painter, const QStyleOptionGraphicsItem
       painter->setFont( messageFont );
       painter->setPen( QColor( 255, 255, 255, 255 ) );
       painter->drawText( thisPaintRect, Qt::AlignCenter | Qt::AlignHCenter, tr( "Rendering map" ) );
-      if ( !mPainterJob && !mDrawingPreview )
+      if ( mPainterJob && mCacheInvalidated && !mDrawingPreview )
+      {
+        // current job was invalidated - start a new one
+        mPreviewScaleFactor = QgsLayoutUtils::scaleFactorFromItemStyle( style );
+        mBackgroundUpdateTimer->start( 1 );
+      }
+      else if ( !mPainterJob && !mDrawingPreview )
       {
         // this is the map's very first paint - trigger a cache update
         mPreviewScaleFactor = QgsLayoutUtils::scaleFactorFromItemStyle( style );