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Alembic Procedural: only subdivide if subsurf modifier is present

As subdivision objects are first class citizens in Alembic, to
differentiate them with non-subdivided polygon meshes, the Alembic
Procedural automatically sets up subdivision properties on the generated
Cycles Mesh.

However, for real-time playback subdivision is far too slow, so this
modifies the detection of a MeshSeqCache modifier used to activate the
procedural to allow for a Subsurf modifier right after the cache one. If
present, the procedural will tag the object for subdivision, if absent, the
object will be treated as a regular mesh.

This is a temporary measure for until subdivision surface settings are part
of the Mesh datablock (see T68891).

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D11162