Plotter: Ensure group style is respected in SVG

Graphics Changed is cleared when setting graphics for a group but needs
to be reset when the group ends.

Fixes: lp:1830245
* https://bugs.launchpad.net/kicad/+bug/1830245

(cherry picked from commit 1a56dfe342)

common/plotters/SVG_plotter.cpp

@@ -303,6 +303,8 @@ void SVG_PLOTTER::StartBlock( void* aData )
 void SVG_PLOTTER::EndBlock( void* aData )
 {
     fprintf( outputFile, "</g>\n" );
+
+    m_graphics_changed = true;
 }