Add Mask::take method

[valadaptive]

I accidentally pushed this to main (reverted it now); sorry about that.

It's nice to be able to create a Pixmap or Mask, draw into it, and then take its raw data back directly without having to do any cloning; this is a common pattern when interoperating with other libraries that have their own bitmap types.

[LaurenzV]

Isn't there already a take method on Pixmap?

[LaurenzV]

I guess it doesn't exist for Mask, but then it should probably also be called take for consistency.

[valadaptive]

I've removed Pixmap::into_data and renamed the method to Mask::take. I've also moved the take methods on Pixmap to be just below data and data_mut--I can't remember if rustdoc sorts alphabetically, but if not, that should make them easier to find.

[LaurenzV]

LGTM, but the PR should be renamed to reflect the new changes.

[valadaptive]

Renamed it.

[LaurenzV]

Ideally shifting the methods should happen in a separate PR, but I'll leave that to you.