h264: use externally provided dimensions when they appear to be more correct.

This is based on:
	commit 30f51509
	Author: Mans Rullgard <mans@mansr.com>
    		h264: allow cropping to AVCodecContext.width/height

The above commit is not taken as is as its buggy (incorrect handling of
the interlaced case) and didnt apply cleanly.

Fixes ticket156
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -2982,9 +2982,19 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
                    "Cannot (re-)initialize context during parallel decoding.\n");
             return -1;
         }
-        avcodec_set_dimensions(s->avctx, s->width, s->height);
-        s->avctx->width  -= (2>>CHROMA444)*FFMIN(h->sps.crop_right, (8<<CHROMA444)-1);
-        s->avctx->height -= (1<<s->chroma_y_shift)*FFMIN(h->sps.crop_bottom, (16>>s->chroma_y_shift)-1) * (2 - h->sps.frame_mbs_only_flag);
+        if(   FFALIGN(s->avctx->width , 16                                 ) == s->width
+           && FFALIGN(s->avctx->height, 16*(2 - h->sps.frame_mbs_only_flag)) == s->height
+           && !h->sps.crop_right && !h->sps.crop_bottom
+           && (s->avctx->width != s->width || s->avctx->height && s->height)
+        ) {
+            av_log(h->s.avctx, AV_LOG_DEBUG, "Using externally provided dimensions\n");
+            s->avctx->coded_width  = s->width;
+            s->avctx->coded_height = s->height;
+        } else{
+            avcodec_set_dimensions(s->avctx, s->width, s->height);
+            s->avctx->width  -= (2>>CHROMA444)*FFMIN(h->sps.crop_right, (8<<CHROMA444)-1);
+            s->avctx->height -= (1<<s->chroma_y_shift)*FFMIN(h->sps.crop_bottom, (16>>s->chroma_y_shift)-1) * (2 - h->sps.frame_mbs_only_flag);
+        }
         s->avctx->sample_aspect_ratio = h->sps.sar;
         av_assert0(s->avctx->sample_aspect_ratio.den);