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

@@ -901,11 +901,20 @@ 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);