Added option to center image when loading particle emission mask

[bgie]

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 #16411

[bgie]

First contribution to the project, so do let me know if I missed some guideline, would be happy to fix it.

[KoBeWi]

Having a separate section for each checkbox is a bit awkward.

I think Centered and Capture from Pixel should be under one section, although the latter might need to be renamed. Something like

Options

• Centered
• Capture Colors from Pixel

[Calinou]

@bgie It looks like you force-pushed your branch in a way that makes it have no commits, which means GitHub automatically closed the pull request.

You need to open a new pull request with a different branch name – do not use your fork's master branch as your branch for pull requests.

[bgie]

@Calinou Thanks for the clarification and clear instructions.
I had noticed I was working on master and tried to correct that, not considering that the pull request is apparently (obviously) linked to a branch name.