gui: avoid a flashing bright window during startup

The very first paint event causes a bright flash to appear before the
configured colors are applied. Swap the uninitialized foreground and
background colors to avoid a blinding flash during startup.

src/gui/shell.h

@@ -208,8 +208,8 @@ private slots:
 
 	// highlight fg/bg - from redraw:highlightset - by default we
 	// use the values from above
-	QColor m_hg_foreground{ Qt::black };
-	QColor m_hg_background{ Qt:: white };
+	QColor m_hg_foreground{ Qt::white };
+	QColor m_hg_background{ Qt::black };
 	QColor m_hg_special;
 
 	/// Modern 'ext_linegrid' highlight definition map

src/gui/shellwidget/shellwidget.h

@@ -197,8 +197,8 @@ public slots:
 	ShellContents m_contents{ 0, 0 };
 	QSize m_cellSize;
 	int m_ascent;
-	QColor m_bgColor{ Qt::white };
-	QColor m_fgColor{ Qt::black };
+	QColor m_bgColor{ Qt::black };
+	QColor m_fgColor{ Qt::white };
 	QColor m_spColor;
 	int m_lineSpace{ 0 };
 	bool m_isLigatureModeEnabled{ false };