Add present mode setters on Pixels

[Kanabenki]

Closes #372.

Add set_present_mode and enable_vsync methods on Pixels. The documentation is almost identical to the corresponding methods on PixelsBuilder since they do the same thing but after the instance has been created.

[parasyte]

Looks good.

Do you think you would ever want a getter for the present mode? It could save the caller from maintaining that state (since Pixels already stores it) and it's just a one-liner. All good if you don't think it's useful.

[Kanabenki]

Good idea, added that!

[parasyte]

Thank you!