Add useful Altivec types, taken from with permission from x264 authors

Originally committed as revision 7125 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ppc/types_altivec.h

+/*
+ * Copyright (c) 2006 Guillaume Poirier <gpoirier@mplayerhq.hu>
+ *
+ * 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
+ */
+
+/***********************************************************************
+ * Vector types
+ **********************************************************************/
+#define vec_u8_t  vector unsigned char
+#define vec_s8_t  vector signed char
+#define vec_u16_t vector unsigned short
+#define vec_s16_t vector signed short
+#define vec_u32_t vector unsigned int
+#define vec_s32_t vector signed int
+
+/***********************************************************************
+ * Null vector
+ **********************************************************************/
+#define LOAD_ZERO const vec_u8_t zerov = vec_splat_u8( 0 )
+
+#define zero_u8v  (vec_u8_t)  zerov
+#define zero_s8v  (vec_s8_t)  zerov
+#define zero_u16v (vec_u16_t) zerov
+#define zero_s16v (vec_s16_t) zerov
+#define zero_u32v (vec_u32_t) zerov
+#define zero_s32v (vec_s32_t) zerov