mpegvideo: allocate sufficiently large scratch buffer for interlaced vid

MPV_decode_mb_internal needs 3 * 16 * linesize bytes of scratch buffer

For interlaced content, linesize is multiplied by two after the allocation
of the scratch buffer, and the dest_cr pointer ends past the buffer.

This patch makes ff_mpv_frame_size_alloc allocate a total of
(aligned line_size) * 2 * 16 * 3 bytes, which suffices even for the
interlaced case.

CC:libav-stable@libav.org

Signed-off-by: Jindrich Makovicka <makovick@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>