avfilter/vf_removegrain: add x86 and x86_64 SSE2 functions

Speed of all modes increased by a factor between 7.4 and 19.8 largely depending
on whether bytes are unpacked into words.  Modes 2, 3, and 4 have been sped-up
by a factor of 43 (thanks quick sort!)

All modes are available on x86_64 but only modes 1, 10, 11, 12, 13, 14, 19, 20,
21, and 22 are available on x86 due to the number of SIMD registers used.

With a contribution from James Almer <jamrial@gmail.com>

LICENSE.md

@@ -16,6 +16,7 @@ Specifically, the GPL parts of FFmpeg are:
 - optional x86 optimizations in the files
   - `libavcodec/x86/flac_dsp_gpl.asm`
   - `libavcodec/x86/idct_mmx.c`
+  - `libavfilter/x86/vf_removegrain.asm`
 - libutvideo encoding/decoding wrappers in
   `libavcodec/libutvideo*.cpp`
 - the X11 grabber in `libavdevice/x11grab.c`