lavc: do not force the emu edge flag

The default get_buffer2() implementation (and possibly some
user ones) does not allocate edges when this flag is set, which may
expose bugs in some decoders. Until the 10 release is out, it is safer
to remove this part.
swscale: fix stride used in planarToNv12Wrapper()

Signed-off-by: Anton Khirnov <anton@khirnov.net>
swscale: add nv12/nv21->yuv420 converter

Signed-off-by: Anton Khirnov <anton@khirnov.net>
swscale/x86/rgb2rgb: add support for AVX

This does not yet include any actual AVX code

Signed-off-by: Anton Khirnov <anton@khirnov.net>
rgb2rgb_template: add MMX/SSE2/AVX-optimized deinterleaveBytes

Signed-off-by: Anton Khirnov <anton@khirnov.net>