riff: chunks must be at 2 byte boundary

Fixes #2244.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
ff_end_tag: assert that the start is aligned.

If its start is not aligned then aligning its end will
likely break many demuxers as they check the size and not
the position.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
dxva2: Prevent redefinition of _WIN32_WINNT

Don't redefine _WIN32_WINNT when targeting Win7/8

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>