shiny/driver/mtldriver: select on receiving from newWindowCh

This is a fixup to CL 208877. The intention there was to select
between receiving a value from the newWindowCh or done channels.
Instead, it was receiving only on the newWindowCh channel,
before proceeding to select between sending to newWindowCh
and receiving from done. That means a shiny program that doesn't
open a window could never exit. This change fixes that.

Change-Id: I1c09ea5f7bec6c9714f20428fdd90d1b756f515d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/209257
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

shiny/driver/mtldriver/mtldriver.go

@@ -71,7 +71,8 @@ func main(f func(screen.Screen)) error {
 	// TODO(dmitshur): Delete this when https://github.com/go-gl/glfw/issues/262 is resolved.
 	// Wait for first window request (or done) before entering main
 	// loop to work around https://github.com/glfw/glfw/issues/1543.
-	case newWindowCh <- <-newWindowCh:
+	case w := <-newWindowCh:
+		newWindowCh <- w
 	case <-done:
 	}
 	for {