Unnecessary computations for SPATIAL INDEX

In InnoDB, spatial index (R-tree) records always start with a field whose length is DATA_MBR_LEN. This field is unfortunately declared as variable-length, so its length is always encoded.

The InnoDB functions dict_index_copy_rec_order_prefix() and rec_copy_prefix_to_buf() are treating spatial indexes as if they could have any number of fields or any length for the first field.