[skwasm] Reify the SkPicture pointer as the right type.

[eyebrowsoffire]

When recreating this unique_ptr, we need to ensure it matches the type in Surface::renderPictures which is released. See here:

  std::unique_ptr<sk_sp<SkPicture>[]> picturePointers =
      std::make_unique<sk_sp<SkPicture>[]>(count);
  for (int i = 0; i < count; i++) {
    picturePointers[i] = sk_ref_sp(pictures[i]);
  }
  // Releasing picturePointers here and will recreate the unique_ptr on the
  // other thread See surface_renderPicturesOnWorker
  skwasm_dispatchRenderPictures(_thread, this, picturePointers.release(), count,
                                callbackId);

[flutter-dashboard]

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[eyebrowsoffire]

This is a subtle memory issue that may have been exposed if we had been running our unit tests against a debug build of skwasm. I have filed flutter/flutter#146516 as a follow-on to get better coverage here.