gpu.c: get_non_parallel_subtree_filters: optionally select final subtrees
gpu.c: get_non_parallel_subtree_filters: optionally select final subtrees

This will allow isolate_permutable_subtrees to separate out
both initial and final subtrees in an upcoming commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/ppcg,Meinersbur/ppcg