background.js: fix updating a BackgroundSource on monitor changes

It was assumed that BackgroundSource objects were always destroyed
on monitor changes because the BackgroundManager objects that hold
references were destroyed, but sequencing of updating of different
BackgroundManager objects meant that was not the case.

Properly update any cached Background objects held by the
BackgroundSource on a change to the monitor layout; in particular this
means updating animations in case they are multi-resolution.