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.
Sybren A. Stüvel