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.

nil_renderer.go

@@ -5,5 +5,6 @@ type nilRenderer struct{}
 func (n nilRenderer) start()              {}
 func (n nilRenderer) stop()               {}
 func (n nilRenderer) write(v string)      {}
+func (n nilRenderer) repaint()            {}
 func (n nilRenderer) altScreen() bool     { return false }
 func (n nilRenderer) setAltScreen(v bool) {}