Differentiate within/approx SpatialPrefixTree leaf cells [LUCENE-6362]

This patch enables indexed cells to be differentiated between those that are completely within the shape that was indexed, and those on the edge (intersects, not within) but reached a precision threshold. The point of this differentiation is to be used by the two-phase optimized predicates in #6641 to be able to avoid an expensive geometry check for accuracy, in more cases than it is able to without this differentiation. In particular, if the query shape is _within_ an indexed shape, then this cell differentiation will enable a predicate to observe that without doing a geometry check.

_note:_ patch here is moved from #6641 as this is really an add-on

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Migrated from [LUCENE-6362](https://issues.apache.org/jira/browse/LUCENE-6362) by David Smiley (@dsmiley)
Attachments: [LUCENE-6362 SPT leaf differentiate.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6362/LUCENE-6362%20SPT%20leaf%20differentiate.patch)
Linked issues:
- #6641