avcodec/cbs_av1: fix storage size for segmentation_params feature_value fields

The valid range is -255 to 255.
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs_av1.h

@@ -210,7 +210,7 @@ typedef struct AV1RawFrameHeader {
     uint8_t segmentation_temporal_update;
     uint8_t segmentation_update_data;
     uint8_t feature_enabled[AV1_MAX_SEGMENTS][AV1_SEG_LVL_MAX];
-    uint8_t feature_value[AV1_MAX_SEGMENTS][AV1_SEG_LVL_MAX];
+    int16_t feature_value[AV1_MAX_SEGMENTS][AV1_SEG_LVL_MAX];
 
     uint8_t delta_q_present;
     uint8_t delta_q_res;