fix: roadmark to boundary conversion

this pr changes the conversion logic of multi-roadmarks in a lane to boundary points. old logic: e.g. A roadmark in a lane with soffset = 0.5, Another roadmark in the same lane has soffset = 100, and if now the lane_marking_distance is set to 1.0 m, the resulted boundary points for first roadmark would be [0], which only contains one points, making the boundary unable to be downsampled.

new logic: same roadmarks in a lane with same lane_marking_distance as above, the resulted boundary points for first roadmark would be [0, 0.5], an additional end point is added to the data structure, making it able to be downsampled.