Support 'correction-of-prediction' encoding for linestrings

Nobody has claimed this yet.

I am suggesting a new encoding type for linestrings which is smaller for objects that have minimal curvature, such as roads which generally continue in their previous direction.

The current approach is to encode the first point and then the delta for the following points. I tested a slightly different approach that is ~17% smaller (at least on the data I used). The idea is as follows. The first point is encoded in full, the second point is encoded as the delta of the first, and then all subsequent points are encoded as a 'correction-of-prediction'.

The first two points are identical to the current approach. To calculate the 3rd point (and subsequent points), a vector is drawn from the first to second point, that vector is then added to the second point(this is the prediction of where the 3rd point should be) and then the value that is encoded is used to correct the prediction. Mathematically

p1: encoded as (x1, y1), calculated as (x1, y1)
p2: encoded as (dx1, dy1), calculated as (x1 + dx1, y1 + dy1)
p3: encoded as (dx2, dy2), calculated as p2 + (p2-p1) + (dx2, dy2)

This works best for roads because they generally continue in their previous direction.

**Next step:**
Test this encoding on a larger dataset and compare results to current encoding.

Start by reviewing the current linestring encoding described in the issue, then test the proposed correction-of-prediction encoding on a larger dataset. Compare its size with the current encoding and use those results to determine whether the proposed encoding should be added.

Written by the indexing model from the issue text.