Make RANSAC Homography estimation faster when many outliers present

The code is minimal but it has a great impact in the spedd of RANSAC homography estimation.

In a nutsell, RANSAC homography estimation time depends both on number of iterations and time needed to fit a homography to a 4 points minimal set and check for inliers. However, it most of the random 4 points sets are not geometrically consistent and check for inliers is not even necesary. 

The code has a heavy impact on estimation time mainly when the number of matches point pairs are outliers is high.

The idea is explained in the paper:

"Speeding-up homography estimation in mobile devices"
Journal of Real-Time Image Processing. 2013. DOI: 10.1007/s11554-012-0314-1
Pablo Márquez-Neila, Javier López-Alberca, José M. Buenaposada, Luis Baumela