avcodec: increase FF_INPUT_BUFFER_PADDING_SIZE to 32

Sometimes the input buffers get directly used as raw images and
SIMD optimized video/image filters can sometimes read more than 16 bytes
over the end.
a specific example is the AVX 24bpp to yuv code

This also fixes fate-vsynth3-rgb

Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -614,7 +614,7 @@ typedef struct AVCodecDescriptor {
  * Note: If the first 23 bits of the additional bytes are not 0, then damaged
  * MPEG bitstreams could cause overread and segfault.
  */
-#define FF_INPUT_BUFFER_PADDING_SIZE 16
+#define FF_INPUT_BUFFER_PADDING_SIZE 32
 
 /**
  * @ingroup lavc_encoding