doc: make x11grab examples consistent with option names

Change -r to -framerate, but either will work.
Change -s to -video_size, but either will work.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
avcodec/x86/dsputil_init: fix cpu flag checks

Fixes linking failure with --disable-sse2

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>