vd_lavc: fix prepare_decoding() failure modes

prepare_decoding() returned a bool that was supposed to tell whether
decoding could work, or if something was fucked. After recent changes to
the decoder loop, this did not work anymore, and caused an endless loop.
Redo it, so it makes more sense. avctx being NULL (software fallback
initialization failed) now signals EOF. hwdec_failed needs to be handled
on send_packet() only, where it probably never happens anyway.

(Who was the idiot who made libavcodec have two entrypoints for
decoding? Oh right, it was me. PEBKAC.)

video/decode/vd_lavc.c

@@ -927,14 +927,14 @@ fallback:
     return avcodec_default_get_buffer2(avctx, pic, flags);
 }
 
-static bool prepare_decoding(struct mp_filter *vd)
+static void prepare_decoding(struct mp_filter *vd)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
     AVCodecContext *avctx = ctx->avctx;
     struct vd_lavc_params *opts = ctx->opts;
 
-    if (!avctx || ctx->hwdec_failed)
-        return false;
+    if (!avctx)
+        return;
 
     int drop = ctx->framedrop_flags;
     if (drop == 1) {
@@ -950,8 +950,6 @@ static bool prepare_decoding(struct mp_filter *vd)
 
     if (ctx->hwdec_request_reinit)
         reset_avctx(vd);
-
-    return true;
 }
 
 static void handle_err(struct mp_filter *vd)
@@ -977,8 +975,13 @@ static int send_packet(struct mp_filter *vd, struct demux_packet *pkt)
     if (ctx->num_requeue_packets && ctx->requeue_packets[0] != pkt)
         return AVERROR(EAGAIN); // cannot consume the packet
 
-    if (!prepare_decoding(vd))
-        return AVERROR_UNKNOWN;
+    if (ctx->hwdec_failed)
+        return AVERROR(EAGAIN);
+
+    if (!ctx->avctx)
+        return AVERROR_EOF;
+
+    prepare_decoding(vd);
 
     if (avctx->skip_frame == AVDISCARD_ALL)
         return 0;
@@ -1019,8 +1022,10 @@ static int decode_frame(struct mp_filter *vd)
     vd_ffmpeg_ctx *ctx = vd->priv;
     AVCodecContext *avctx = ctx->avctx;
 
-    if (!prepare_decoding(vd))
-        return AVERROR_UNKNOWN;
+    if (!avctx)
+        return AVERROR_EOF;
+
+    prepare_decoding(vd);
 
     // Re-send old packets (typically after a hwdec fallback during init).
     if (ctx->num_requeue_packets)