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.

standard_renderer.go

@@ -202,6 +202,10 @@ func (r *standardRenderer) write(s string) {
 	_, _ = r.buf.WriteString(s)
 }
 
+func (r *standardRenderer) repaint() {
+	r.lastRender = ""
+}
+
 func (r *standardRenderer) altScreen() bool {
 	return r.altScreenActive
 }