Improve readability of 4bpp raw decoder and prepare for supporting 2bpp.

Originally committed as revision 21026 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/rawdec.c

@@ -118,12 +118,13 @@ static int raw_decode(AVCodecContext *avctx,
     if(avctx->bits_per_coded_sample == 4 && avctx->pix_fmt==PIX_FMT_PAL8 &&
        (!avctx->codec_tag || avctx->codec_tag == MKTAG('r','a','w',' '))){
         int i;
-        for(i=256*2; i+1 < context->length>>1; i++){
-            context->buffer[2*i+0]= buf[i-256*2]>>4;
-            context->buffer[2*i+1]= buf[i-256*2]&15;
+        uint8_t *dst = context->buffer + 256*4;
+        buf_size = context->length - 256*4;
+        for(i=0; 2*i+1 < buf_size; i++){
+            dst[2*i+0]= buf[i]>>4;
+            dst[2*i+1]= buf[i]&15;
         }
-        buf= context->buffer + 256*4;
-        buf_size= context->length - 256*4;
+        buf= dst;
     }
 
     if(buf_size < context->length - (avctx->pix_fmt==PIX_FMT_PAL8 ? 256*4 : 0))