Accumulated error with relative coordinates in ST_AsSvg

[nicklas]

When using ST_ASSvg outputting relative rounded coordinates you might get accumulated rounding errors.

I saw huge errors like that when testing twkb on polygons with thousands of vertex-points.

Example:

select 
st_assvg('LINESTRING(0 0, 0.4 0, 0.8 0, 1.2 0,1.6 0, 2 0)'::geometry,1,0)

returns:

M 0 0 l 0 0 0 0 0 0 0 0 0 0

I fixed it in twkb-case in the spike with #11165, probably the same approach can be used here.

Just not get the relative value as relative to the previous coordinate, but relative to the accumulates relative coordinates achieved.