Improve Seam Optimization in Enblend to allow fewer "missing heads" and broken lines.

Improve the way seams are chosen when blending images together to prevent broken lines and half-people. Enblend does not currently work very well if you compare it to other blenders available. Even Smartblend 1.25 will usually do a better job at seam optimization than the newest Enblend (Smartblend 1.25 is 5 years old). The "Antighost" in Autopano Pro / Giga is even better, and 5-10 times faster.