h264: ensure that get_format() is called when changing format but not otherwise.

Fixes Ticket2288

Tested-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Fix Win64 AVX h264_deblock by not using redzone on Win64

Thanks-to: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>