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 r11165, 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.