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>

libavfilter/removegrain.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2015 Paul B Mahol
+ * Copyright (c) 2015 James Darnley
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "avfilter.h"
+
+typedef struct RemoveGrainContext {
+    const AVClass *class;
+
+    int mode[4];
+
+    int nb_planes;
+    int planewidth[4];
+    int planeheight[4];
+    int skip_even;
+    int skip_odd;
+
+    int (*rg[4])(int c, int a1, int a2, int a3, int a4, int a5, int a6, int a7, int a8);
+
+    void (*fl[4])(uint8_t *dst, uint8_t *src, ptrdiff_t stride, int pixels);
+} RemoveGrainContext;
+
+void ff_removegrain_init_x86(RemoveGrainContext *rg);