refactored approxpoly
convex hull converted to C++; other 2 functions in convhull.cpp are yet to be finished.
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++
converted fitline to C++, fixed Mat::checkVector().
converted few more comp. geometry functions to C++
fixed warnings; restored fixed_size parameter in AutoBuffer
Merge pull request #317 from vpisarev:c2cpp_refactor_imgproc