Issue #3618813: Fill the background for sources with transparency

Closes #3618813

Adds a setting that fills the background of images that have a transparent one, instead of leaving them on black.

The configured background type is not used for those images: a tiled or blurred copy of a logo is a smeared copy of the logo, not a background. Images without a transparent background are unaffected.

Existing image styles are pinned to the previous behaviour by a post update, so nothing a site already produces changes on upgrade. The new default applies to effects added afterwards.