Update for compatibility with libx265 4.0

libx265 4.0 has changed the signature of `x265_api::encoder_encode` and `x265_encoder_encode` functions to accept a pointer to an array of pointers to output pictures. That pointer is now required to be non-null, otherwise the encode call crashes.

Upstream bug report: https://bitbucket.org/multicoreware/x265_git/issues/952/crash-in-libheif-tests

libheif/plugins/encoder_x265.cc

@@ -901,12 +901,21 @@ static struct heif_error x265_encode_image(void* encoder_raw, const struct heif_
 
  encoder->encoder = api->encoder_open(param);
 
+#if X265_BUILD >= 212
+ x265_picture* out_pic = NULL;
+ api->encoder_encode(encoder->encoder,
+ &encoder->nals,
+ &encoder->num_nals,
+ pic,
+ &out_pic);
+#else
  api->encoder_encode(encoder->encoder,
  &encoder->nals,
  &encoder->num_nals,
  pic,
  NULL);
-
+#endif
+
  api->picture_free(pic);
  api->param_free(param);