Animation data (including drivers) on inactive view layers now work. The
removal of such view layers was too optimistic; they are now kept
around. The bases are still removed, mostly for safety sake and to keep
the changes to a minimum.
`scene_remove_unused_view_layers()` has been renamed to
`scene_minimize_unused_view_layers()` to reflect its new functionality.
For compatibility with assumptions in other areas of the code, the
function still ensures the input view layer is at index 0.
This also introduces a new function
`BKE_view_layer_free_object_content(view_layer)`, which is a subset of
the functionality of `BKE_view_layer_free()`.
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