vdpau: fix surface leaks if the picture is not extracted

This can happen in some corner cases such as flushing (luckily the leak
would automatically be resolved when the device instance is released).

modules/hw/vdpau/avcodec.c

@@ -81,26 +81,41 @@ static int Lock(vlc_va_t *va, AVFrame *ff)
         return VLC_EGENERIC;
     }
 
+    vlc_vdp_video_field_t *field = vlc_vdp_video_create(sys->vdp, surface);
+    if (unlikely(field == NULL))
+        return VLC_ENOMEM;
+
     ff->data[0] = (void *)sys->vdp; /* must be non-NULL */
     ff->data[3] = (void *)(uintptr_t)surface;
-    ff->opaque = (void *)(uintptr_t)surface;
+    ff->opaque = field;
     return VLC_SUCCESS;
 }
 
 static void Unlock(vlc_va_t *va, AVFrame *ff)
 {
-    (void) va;
+    vlc_vdp_video_field_t *field = ff->opaque;
+
+    assert(field != NULL);
+    field->destroy(field);
+
     ff->data[0] = ff->data[3] = NULL;
     ff->opaque = NULL;
+    (void) va;
 }
 
 static int Copy(vlc_va_t *va, picture_t *pic, AVFrame *ff)
 {
-    vlc_va_sys_t *sys = va->sys;
-    VdpVideoSurface surface = (uintptr_t)ff->opaque;
+    vlc_vdp_video_field_t *field = ff->opaque;
+
+    assert(field != NULL);
+    field = vlc_vdp_video_copy(field);
+    if (unlikely(field == NULL))
+        return VLC_ENOMEM;
 
-    return vlc_vdp_video_attach(sys->vdp, surface, pic)
-        ? VLC_ENOMEM : VLC_SUCCESS;
+    assert(pic->context == NULL);
+    pic->context = field;
+    (void) va;
+    return VLC_SUCCESS;
 }
 
 static int Init(vlc_va_t *va, void **ctxp, vlc_fourcc_t *chromap,