[FEATURE] Ability (option) to change Acrylic Background Source

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# Description of the new feature/enhancement

Ability to change `AcrylicBrush.BackgroundSource`. It will be really useful when someone wants to set an image background and enable Acrylic *over* the image (instead of the app background), and hence make a great feature! I personally love AcrylicBrush and have some images that suit acrylic really well, but can't use the Terminal acrylic on those background images since the BackgroundSource is different (`AcrylicBrush.BackgroundSource.HostBackdrop`, I believe). Adding an option in the settings to change the said background source of the acrylic brush (to lay it over the background image and not the app background/desktop) would be a gamechanger (at least in my humble opinion).

# Proposed technical implementation details (optional)

An ability to toggle(?) `AcrylicBrush.BackgroundSource` between `AcrylicBrush.BackgroundSource.Backdrop` and `AcrylicBrush.BackgroundSource.HostBackdrop` (if I remember the valid options/choices correctly) through the Terminal settings.
Just a normal setting (maybe in the `Appearance` tab?) should be enough. Just two `RadioButtons` or a `ComboBox` to choose an acrylic background source. And of course, an `"acrylicBackgroundSource"` property whose possible values are `"backdrop"` and `"hostBackdrop"` (or whatever values you want here, whichever may be more understandable).

Please let me know if you want more details for a better understanding. I'm ready to explain.

Thank you!

Start with the Terminal settings, especially the proposed Appearance tab, and trace how AcrylicBrush.BackgroundSource is selected. Add a setting that switches between Backdrop and HostBackdrop, then verify that the selected source is applied when Acrylic is used over an image background.

Written by the indexing model from the issue text.