Fix T77754: Crash after any alembic import undo in an empty scene

Thanks @mont29 for this patch.

This creates an explicit undo step after the Alembic importer has finished
running. This is necessary when the importer runs as a background job.

Fix T77712: Crash on proxied rig, custom bone shape, driver targeting rig

It turns out that
`DepsgraphNodeBuilder::build_object_data_geometry(Object *object, bool
is_object_visible)` was called for the custom shape with
`is_object_visible=false` when there are drivers, and
`is_object_visible=true` when there aren't any.

Fix missing relation in compositor depsgraph

This is a fix for c7694185c92. An object without base can still be in the
depsgraph, and then the `VIEW_LAYER_EVAL` node does not exist.

This popped up while @Sergey was looking into T78264.

Alembic exporter: Fix Windows build errors

Some Windows-specific code needed adjustment after 2917df21adc8a1ce.

Alembic: new exporter based on the USD exporter structure

The Alembic exporter has been restructured by leverages the
`AbstractHierarchyIterator` introduced by the USD exporter. The produced
Alembic files have not changed much (details below), as the Alembic
writing code has simply been moved from the old exporter to the new. How
the export hierarchy is handled changed a lot, though, and also the way
in which transforms are computed. As a result, T71395 is fixed.

Differences between the old and new exporter, in terms of the produced
Alembic file:
- Duplicated objects now have a unique numerical suffix.
- Matrices are computed differently, namely by simply computing the
evaluated transform of the object relative to the evaluated transform
of its export-parent. This fixes {T71395}, but otherwise should
produce the same result as before (but with simpler code).

Compared to the old Alembic exporter, Subdivision modifiers are now
disabled in a cleaner, more efficient way (they are disabled when
exporting with the "Apply Subdivisions" option is unchecked). Previously
the exporter would move to a new frame, disable the modifier, evaluate
the object, and enable the modifier again. This is now done before
exporting starts, and modifiers are only restored when exporting ends.

Some issues with the old Alembic exporter that have NOT been fixed in
this patch:
- Exporting NURBS patches and curves (see T49114 for example).
- Exporting flattened hierarchy in combination with dupli-objects. This
seems to be broken in the old Alembic exporter as well, but nobody
reported this yet.

Differential Revision: https://developer.blender.org/D7664

Reviewed By: Sergey

Cleanup: Depsgraph, remove unused `Base *` parameter

The `Base *` parameter of `DepsgraphRelationBuilder::build_object()` was
made redundant by c7694185c92aa. This commit actually removes it.

No functional changes.

Revert "Fix T78071: Drivers reading object visibility not updating automatically"

This reverts commit baa0da3e69a1225cd18c075be5563c7d811b5347.

The commit causes some issues I didn't foresee, I'd rather take the time
to do it properly than hastily try and commit a fix for it.

Fix T78071: Drivers reading object visibility not updating automatically

An object can be targeted by a driver that reads its `hide_viewport` or
`hide_render` property. The existence of such a driver will create a
relation between the 'sync base flags' depsgrpah node, and the datablock
containing the driver. When the object is hidden, however, it has no
base, and thus it had no 'sync base flags' depsgraph node. To support
such a driver, that depsgraph node is now always added, but for hidden
objects it will just be a no-op. If the node is not used by anything, it
will be automatically disconnected and have a negligible effect on
performance.

Fix T77712: Crash on proxied rig, custom bone shape, driver targeting rig

It turns out that
`DepsgraphNodeBuilder::build_object_data_geometry(Object *object, bool
is_object_visible)` was called for the custom shape with
`is_object_visible=false` when there are drivers, and
`is_object_visible=true` when there aren't any.

Cleanup: USD, move code from `USD` to `blender::io::usd` namespace

No functional changes.