Doesn't draw the first arc point in debug mode.

This patch fixes it.
Make debug color clearer.

Currently, it's hard to recognize inside distance field. This patch show inside
distance field red color and outside distance filed green color.

In addition, this patch composite other colors on the color of distance field
by source-over.

In addition, following complicated value is meaningless;
abs(gsdist) / max(float(gi.nominal_size.x), float(gi.nominal_size.y))) * 4.
So this patch uses 1. - gudist as alpha, which show which point has distance 1
in |gudist| field.