Avoid wasting 4 cpu cycles per MB in redundantly calculating qp_thresh.

Originally committed as revision 21243 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -2110,6 +2110,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
                 return 1; // deblocking switched inside frame
         }
     }
+    h->qp_thresh= 15 - h->slice_alpha_c0_offset - FFMAX3(0, h->pps.chroma_qp_index_offset[0], h->pps.chroma_qp_index_offset[1]);
 
 #if 0 //FMO
     if( h->pps.num_slice_groups > 1  && h->pps.mb_slice_group_map_type >= 3 && h->pps.mb_slice_group_map_type <= 5)

libavcodec/h264.h

@@ -273,6 +273,8 @@ typedef struct H264Context{
 
     int chroma_qp[2]; //QPc
 
+    int qp_thresh;      ///< QP threshold to skip loopfilter
+
     int prev_mb_skipped;
     int next_mb_skipped;
 

libavcodec/h264_loopfilter.c

@@ -654,7 +654,7 @@ void ff_h264_filter_mb( H264Context *h, int mb_x, int mb_y, uint8_t *img_y, uint
     //for sufficiently low qp, filtering wouldn't do anything
     //this is a conservative estimate: could also check beta_offset and more accurate chroma_qp
     if(!FRAME_MBAFF){
-        int qp_thresh = 15 - h->slice_alpha_c0_offset - FFMAX3(0, h->pps.chroma_qp_index_offset[0], h->pps.chroma_qp_index_offset[1]);
+        int qp_thresh = h->qp_thresh;
         int qp = s->current_picture.qscale_table[mb_xy];
         if(qp <= qp_thresh
            && (mb_x == 0 || ((qp + s->current_picture.qscale_table[mb_xy-1] + 1)>>1) <= qp_thresh)