cmd/gogio: start using layout.Flex in the e2e app

This vastly simplifies our code, and saves us the ugly math.

While at it, establish that a TestDriver must have a white background,
which is already satisfied by both existing implementations.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/x11_test.go

@@ -44,7 +44,9 @@ func (d *X11TestDriver) Start(t_ *testing.T, path string, width, height int) (cl
 	display := fmt.Sprintf(":%d", rnd.Intn(100000)+1)
 
 	var xprog string
-	xflags := []string{"-wr"}
+	xflags := []string{
+		"-wr", // we want a white background; the default is black
+	}
 	if *headless {
 		xprog = "Xvfb" // virtual X server
 		xflags = append(xflags, "-screen", "0", fmt.Sprintf("%dx%dx24", width, height))