Blender Git Loki

Rename USDStageReader prune functions.

Renamed USDStageReader prune_by_visibility()
and prune_by_purpose() member functions to
include_by_visibility() and include_by_purpse(),
respectively, to reflect the current semantics.
I.e., both functions are now answering the question,
'should the prim be included in the traversal',
returning true to include and false to exclude.