avcodec/imgconvert: Fix loss mask bug in avcodec_find_best_pix_fmt_of_list()

example:

AVPixelFormat pixFmts[] = { AV_PIX_FMT_RGB24, AV_PIX_FMT_RGBA };
int loss = 0;
AVPixelFormat best = avcodec_find_best_pix_fmt_of_list(pixFmts, AV_PIX_FMT_BGRA, 1, &loss);

best is AV_PIX_FMT_RGB24. But AV_PIX_FMT_RGBA is better.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 354b26a394)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/imgconvert.c

@@ -69,10 +69,14 @@ enum AVPixelFormat avcodec_find_best_pix_fmt_of_list(const enum AVPixelFormat *p
     int i;
 
     enum AVPixelFormat best = AV_PIX_FMT_NONE;
+    int loss;
 
-    for(i=0; pix_fmt_list[i] != AV_PIX_FMT_NONE; i++)
-        best = avcodec_find_best_pix_fmt_of_2(best, pix_fmt_list[i], src_pix_fmt, has_alpha, loss_ptr);
+    for (i=0; pix_fmt_list[i] != AV_PIX_FMT_NONE; i++) {
+        loss = *loss_ptr;
+        best = avcodec_find_best_pix_fmt_of_2(best, pix_fmt_list[i], src_pix_fmt, has_alpha, &loss);
+    }
 
+    *loss_ptr = loss;
     return best;
 }