Purpose: qsvenc: make sure that interlaced encoding works.  Also,
reduce the vertical alignment constraint when possible to reduce
memory usage.

Note: Most of this code used to be present in ffmpeg and was
eliminated in revision 1f26a231 on Oct. 31, 2016 (qsv: Merge libav
implementation, at
https://github.com/FFmpeg/FFmpeg/commit/1f26a231bb065276cd80ce02957c759f3197
edfa#diff-7d84a34d58597bb7aa4b8239dca1f9f8).  Already applied to
libav.
Reviewed-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit 8fd8f91e)
Signed-off-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Aaron Levinson <alevinsn@aracnet.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>