surject: check anchor sliding on target paths

[Sagorikanag]

Changelog Entry

• vg surject --max-slide now detects slideable anchors using both the
  read sequence and the target-path sequence.

Description

The existing --max-slide check pruned an anchor when the same sequence
occurred at a nearby offset in the read. However, an anchor can also be
ambiguous because its reference sequence occurs again nearby on the path
being surjected to.

This change preserves the existing read-based check and adds a target-path
check. For every offset within --max-slide:

1. If the anchor repeats at that offset in the read, it is pruned immediately.
2. Otherwise, the same offset is checked on the current target path.
3. If the reference sequence repeats there, the anchor is pruned.

The target-path comparison is limited to the path currently being surjected
to. An anchor occurring on other graph paths does not affect this decision.
Both forward and reverse-strand anchors are handled.