expand method
Implementation
void expand(Vector4 v1, Vector4 v2, double pixels) {
num x = v2.x - v1.x;
num y = v2.y - v1.y;
num det = x * x + y * y;
if (det == 0) return;
num idet = pixels / Math.sqrt(det);
x *= idet;
y *= idet;
v2.x += x;
v2.y += y;
v1.x -= x;
v1.y -= y;
}