viewer: makeCurrent before retrieving pixel at cursor

Fixes crash if viewer was somehow made not current (e.g. if the export dialog was shown)

app/widget/viewer/viewerdisplay.cpp

@@ -889,6 +889,8 @@ void ViewerDisplayWidget::EmitColorAtCursor(QMouseEvent *e)
       QPointF pixel_pos = GenerateDisplayTransform().inverted().map(e->pos());
       pixel_pos /= texture_->params().divider();
 
+      makeCurrent();
+
       reference = renderer()->GetPixelFromTexture(texture_.get(), pixel_pos);
       display = color_service()->ConvertColor(reference);
     }