Fix gob.Register

[sago35]

This PR corrects the following errors

$ go run ./examples/basic/
panic: gob: name not registered for interface: "*image.RGBA"

goroutine 35 [running]:
github.com/sago35/tinydisplay.(*Client).update(0x0?)
        /home/sago35/dev/src/github.com/sago35/tinydisplay/client.go:144 +0xd3
github.com/sago35/tinydisplay.(*Client).Tick(0xc000200300)
        /home/sago35/dev/src/github.com/sago35/tinydisplay/client.go:48 +0x5b
created by github.com/sago35/tinydisplay.NewClient
        /home/sago35/dev/src/github.com/sago35/tinydisplay/client.go:41 +0x1a5
exit status 2

[sago35]

It worked fine.