Force a full repaint when resizing the window

v0.13.4 introduced a regression where lines weren't always cleared when
resizing the window resulting in the presence of rendering artifacts.
This commit fixes that.

renderer.go

@@ -5,6 +5,7 @@ type renderer interface {
 	start()
 	stop()
 	write(string)
+	repaint()
 	altScreen() bool
 	setAltScreen(bool)
 }