Restore picture atlas handling

python/core/layout/qgslayoutcontext.sip

@@ -235,6 +235,11 @@ Emitted when the context's ``layer`` is changed.
 %Docstring
 Emitted certain settings in the context is changed, e.g. by setting a new feature or vector layer
 for the context.
+%End
+
+    void dpiChanged();
+%Docstring
+Emitted when the context's DPI is changed.
 %End
 
 };

src/core/layout/qgslayoutcontext.cpp

@@ -89,7 +89,11 @@ void QgsLayoutContext::setLayer( QgsVectorLayer *layer )
 
 void QgsLayoutContext::setDpi( double dpi )
 {
+  if ( dpi == mMeasurementConverter.dpi() )
+    return;
+
   mMeasurementConverter.setDpi( dpi );
+  emit dpiChanged();
 }
 
 double QgsLayoutContext::dpi() const

src/core/layout/qgslayoutcontext.h

@@ -236,6 +236,11 @@ class CORE_EXPORT QgsLayoutContext : public QObject
      */
     void changed();
 
+    /**
+     * Emitted when the context's DPI is changed.
+     */
+    void dpiChanged();
+
   private:
 
     Flags mFlags = nullptr;

src/core/layout/qgslayoutitempicture.cpp

@@ -54,14 +54,12 @@ QgsLayoutItemPicture::QgsLayoutItemPicture( QgsLayout *layout )
 
   //connect some signals
 
-#if 0 //TODO
   //connect to atlas feature changing
   //to update the picture source expression
-  connect( &mComposition->atlasComposition(), &QgsAtlasComposition::featureChanged, this, [ = ] { refreshPicture(); } );
+  connect( &layout->context(), &QgsLayoutContext::changed, this, [ = ] { refreshPicture(); } );
 
   //connect to layout print resolution changing
-  connect( layout->context(), &QgsLayoutContext::printResolutionChanged, this, &QgsLayoutItemPicture::recalculateSize );
+  connect( &layout->context(), &QgsLayoutContext::dpiChanged, this, &QgsLayoutItemPicture::recalculateSize );
-#endif
 
   connect( this, &QgsLayoutItem::sizePositionChanged, this, &QgsLayoutItemPicture::shapeChanged );
 }

tests/src/core/testqgslayoutcontext.cpp

@@ -145,9 +145,17 @@ void TestQgsLayoutContext::layer()
 void TestQgsLayoutContext::dpi()
 {
   QgsLayoutContext context;
+
+  QSignalSpy spyDpiChanged( &context, &QgsLayoutContext::dpiChanged );
   context.setDpi( 600 );
   QCOMPARE( context.dpi(), 600.0 );
   QCOMPARE( context.measurementConverter().dpi(), 600.0 );
+  QCOMPARE( spyDpiChanged.count(), 1 );
+
+  context.setDpi( 600 );
+  QCOMPARE( spyDpiChanged.count(), 1 );
+  context.setDpi( 6000 );
+  QCOMPARE( spyDpiChanged.count(), 2 );
 }
 
 void TestQgsLayoutContext::renderContextFlags()