removed rarely used fixed_size parameter from AutoBuffer type, added optional AutoBuffer* but to cvarrToMat in order to speedup CvSeq->Mat conversion; finished conversion of convex hull and related functions to C++

Changed RANSAC Homography estimation for speed. Each minimal set is checked for consistency before actually fit a candidate homography.

This suppresses "unused variable" CMake warning when user explicilty disables
module unavailable in selected configuration

issue 2526

issue #2678