RV30 decoder passes possible frame sizes in extradata and selects

an appropriate frame size from them in slice, make my decoder do
that as well.

This fixes issue 779

Originally committed as revision 16529 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/rv30.c

@@ -38,6 +38,7 @@ static int rv30_parse_slice_header(RV34DecContext *r, GetBitContext *gb, SliceIn
     int mb_bits;
     int w = r->s.width, h = r->s.height;
     int mb_size;
+    int rpr;
 
     memset(si, 0, sizeof(SliceInfo));
     if(get_bits(gb, 3))
@@ -49,9 +50,14 @@ static int rv30_parse_slice_header(RV34DecContext *r, GetBitContext *gb, SliceIn
     si->quant = get_bits(gb, 5);
     skip_bits1(gb);
     si->pts = get_bits(gb, 13);
-    skip_bits(gb, r->rpr);
+    rpr = get_bits(gb, r->rpr);
+    if(!rpr){
         si->width  = w;
         si->height = h;
+    }else{
+        si->width  = r->s.avctx->extradata[6 + rpr*2] << 2;
+        si->height = r->s.avctx->extradata[7 + rpr*2] << 2;
+    }
     mb_size = ((w + 15) >> 4) * ((h + 15) >> 4);
     mb_bits = ff_rv34_get_start_offset(gb, mb_size);
     si->start = get_bits(gb, mb_bits);
@@ -248,6 +254,10 @@ static av_cold int rv30_decode_init(AVCodecContext *avctx)
     }
     r->rpr = (avctx->extradata[1] & 7) >> 1;
     r->rpr = FFMIN(r->rpr + 1, 3);
+    if(avctx->extradata_size - 8 < (r->rpr - 1) * 2){
+        av_log(avctx, AV_LOG_ERROR, "Insufficient extradata - need at least %d bytes, got %d\n",
+               6 + r->rpr * 2, avctx->extradata_size);
+    }
     r->parse_slice_header = rv30_parse_slice_header;
     r->decode_intra_types = rv30_decode_intra_types;
     r->decode_mb_info     = rv30_decode_mb_info;