Add method to get the `ImageHandle` of a `Sprite`

[Kwarrtz]

Currently, any widget built on top of a Sprite that wants to update the image buffer without allocating a new one or get the size of the image through the DrawShared API has to maintain its own separate copy of the active ImageHandle, which seems unnecessary and potentially error-prone. Adding a method to Sprite that returns the Option<&ImageHandle> would simplify this.

[dhardy]

This should be easy; want to make a PR? Working on a related PR.

[dhardy]

Also fixed in #627.