Fix dashed line algorithm

[KmolYuan]

#483
I'm sorry, I found some problems using the dashed line algorithm. When the distance between points is greater than the size or spacing of the dashed line, the algorithm will produce an incorrect continuous straight line.

This new algorithm was tested using two curve coordinate sequences (as shown in the following), "point distance less than size" and "point distance greater than size," the above results were corrected.

In addition, if the size equals 0 pixel, it will not be drawn.

Left: point distance less than size or spacing
Right: point distance greater than size or spacing

[AaronErhardt]

Looks good, thanks!