Clear screen to black on software renderer after it starts

src/qt/qt_softwarerenderer.cpp

@@ -73,6 +73,12 @@ SoftwareRenderer::render()
     m_backingStore->flush(rect);
 }
 
+void
+SoftwareRenderer::exposeEvent(QExposeEvent* event)
+{
+    render();
+}
+
 void
 SoftwareRenderer::onBlit(int buf_idx, int x, int y, int w, int h)
 {