Added option to center image when loading particle emission mask

[bgie]

I messed up pull request 78583 by working on master and some rebasing/resetting and force pushing. This is a recreation of that pull request with a change as commented by @KoBeWi

The dialog now looks like:

Recap: when loading an image to use as a particle emission mask, the option to center the image is now available. The centering is applied when generating the mask, so no extra runtime cost is involved.

Fixes https://github.com/godotengine/godot/issues/16411