Add support for the Geopoly extension to the R-Tree extension. This also

Add support for the Geopoly extension to the R-Tree extension.  This also
involves adding the SQLITE_INDEX_CONSTRAINT_FUNCTION capability to the
xFindFunction method of the sqlite3_module object, and to the
sqlite3_index_info.aConstraint.op field in the xBestIndex implementation of
virtual tables.

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