vd_lavc: fix device leak with copy-mode hwaccels

Apparently this was broken by the "ctx->hwdec" check in the if condition
guarding the destroy call, and "ctx->hwdec = NULL;" was moved up
earlier, making this always dead code.

This should probably be refcounted or so, although that could make it
worse as well. For now, add a flag whether the device should be
destroyed.

Fixes #4735.

video/decode/lavc.h

@@ -61,6 +61,7 @@ typedef struct lavc_ctx {
 
     // Set by generic hwaccels.
     struct mp_hwdec_ctx *hwdec_dev;
+    bool owns_hwdec_dev;
 
     int hwdec_fmt;
     int hwdec_w;

video/decode/vd_lavc.c

@@ -592,6 +592,7 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
             ctx->hwdec_dev = hwdec_create_dev(vd, ctx->hwdec, false);
             if (!ctx->hwdec_dev)
                 goto error;
+            ctx->owns_hwdec_dev = !!ctx->hwdec->create_dev;
             if (ctx->hwdec_dev->restore_device)
                 ctx->hwdec_dev->restore_device(ctx->hwdec_dev);
             if (!ctx->hwdec->set_hwframes) {
@@ -688,10 +689,10 @@ static void uninit_avctx(struct dec_video *vd)
 
     avcodec_free_context(&ctx->avctx);
 
-    if (ctx->hwdec_dev && ctx->hwdec && ctx->hwdec->generic_hwaccel &&
-        ctx->hwdec_dev->destroy)
+    if (ctx->hwdec_dev && ctx->owns_hwdec_dev && ctx->hwdec_dev->destroy)
         ctx->hwdec_dev->destroy(ctx->hwdec_dev);
     ctx->hwdec_dev = NULL;
+    ctx->owns_hwdec_dev = false;
 
     ctx->hwdec_failed = false;
     ctx->hwdec_fail_count = 0;