added missing contrib & legacy chapters to the reference manual; fixed (x, y)->(y, x) ordering in Mat.at (ticket #1726)

Setting camera parameters on Android 2.2.2 fixed. Old Android does not support continuous-video focus mode.

added explicit VideoWriter::release(). Fixed video stream finalization (write remaining frames); Fixed occasional crash in the case of odd-width or odd-height videos; Fixed a few problems in ffmpeg tests. Positioning still does not work well.