avcodec/proresenc: make transparency honored in mov/QT

In the mov muxer (in mov_write_video_tag()), bits_per_coded_sample will
be written under certain conditions and is required to be 32 for the
transparency to be honored in QuickTime.

prores_kostya already has this setting but prores_anatoliy and
prores_videotoolbox didn't.

libavcodec/proresenc_anatoliy.c

@@ -930,6 +930,9 @@ static av_cold int prores_encode_init(AVCodecContext *avctx)
         }
     }
 
+    if (ctx->need_alpha)
+        avctx->bits_per_coded_sample = 32;
+
     ff_fdctdsp_init(&ctx->fdsp, avctx);
 
     avctx->codec_tag = AV_RL32((const uint8_t*)profiles[avctx->profile].name);

libavcodec/videotoolboxenc.c

@@ -521,6 +521,8 @@ static CMVideoCodecType get_cm_codec_type(AVCodecContext *avctx,
         }
         return kCMVideoCodecType_HEVC;
     case AV_CODEC_ID_PRORES:
+        if (desc && (desc->flags & AV_PIX_FMT_FLAG_ALPHA))
+            avctx->bits_per_coded_sample = 32;
         switch (profile) {
         case AV_PROFILE_PRORES_PROXY:
             return MKBETAG('a','p','c','o'); // kCMVideoCodecType_AppleProRes422Proxy