avformat/movenc: AVC Intra support

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/libx264: add avcintra-class

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/movenc: automatically set AVC-Intra tag

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/mpegvideoencdsp: improve ff_pix_sum16_sse2

~15% faster.

Also add an mmxext version that takes advantage of the new code, and
build it alongside with the mmx version only on x86_32.

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