Specifically, the GPL parts of FFmpeg are
- libpostproc
-- optional MMX optimizations for YUV to RGB colorspace conversion in
- libswscale/x86/yuv2rgb_template.c
- optional x86 optimizations in the files
- libavcodec/x86/h264_deblock_sse2.asm
- libavcodec/x86/h264_idct_sse2.asm
libavcodec/x86/idct_mmx.c
- the X11 grabber in libavdevice/x11grab.c