Fix dropped redraw events (#1675)

If screen.Redraw() is called very quickly after a key or mouse event,
it may send the redraw event while micro is not waiting for it but
still processing the key or mouse event. Since drawChan is non-buffered
and at the same time non-blocking, this redraw event will be simply lost,
so the screen content will not be up-to-date.

cmd/micro/micro.go

@@ -363,6 +363,9 @@ func DoEvent() {
 	case <-shell.CloseTerms:
 	case event = <-events:
 	case <-screen.DrawChan():
+		for len(screen.DrawChan()) > 0 {
+			<-screen.DrawChan()
+		}
 	}
 
 	if action.InfoBar.HasPrompt {