Harden against arbitrary inputs

[bogdanni]

The first three normalize() in drawSegment() of polyline.geom may have trouble with arbitrary input.

I had to define

vec2 safe_normalize(vec2 v)
{
    float len = length(v);
    if (len == 0)
        return vec2(0, 1); // arbitrary
    else
        return v / len;
}

In my observation, ATI drivers do the right thing, NVIDIA and Intel drivers return something unreasonable.

The second pair of normalize() are maybe safe (?).

[vicrucann]

Interesting observation. It never came to my mind the normalize() may not be safe and the behavior depends on the drivers. Although, I never seem to have a problem with the default implementation on my NVIDIA, I assume your suggestion makes total sense to make it safer.