message: Fix inconsistent cube face order.
  KTX / OpenGL are PX,NX,PY,NZ,PZ,NZ.
  Cubemap::Face is NX,PX,NY,PY,NZ,PZ.
  I considered changing the Cubemap::Face enum, but applying ^1 upon
  export seems less intrusive.
diff: @@ -789,7 +789,8 @@ void iblRoughnessPrefilter(const utils::Path& iname,
  if (g_ktxContainer) {
  for (uint32_t j = 0; j < 6; j++) {
  KtxBlobIndex blobIndex {(uint32_t) level, 0, j};
  - LinearImage image = toLinearImage(dst.getImageForFace((Cubemap::Face) j));
  + // Cubemap::Face has a slightly different ordering than GL / KTX, hence the xor.
  + LinearImage image = toLinearImage(dst.getImageForFace((Cubemap::Face) (j^1)));
  auto uintData = fromLinearToRGBM<uint8_t>(image);
  container.setBlob(blobIndex, uintData.get(), dim * dim * 4);
  }
  @@ -931,7 +932,8 @@ void extractCubemapFaces(const utils::Path& iname, const Cubemap& cm, const util
  .pixelDepth = 0,
  };
  for (uint32_t j = 0; j < 6; j++) {
  - LinearImage image = toLinearImage(cm.getImageForFace((Cubemap::Face) j));
  + // Cubemap::Face has a slightly different ordering than GL / KTX, hence the xor.
  + LinearImage image = toLinearImage(cm.getImageForFace((Cubemap::Face) (j^1)));
  auto uintData = fromLinearToRGBM<uint8_t>(image);
  container.setBlob({0, 0, j}, uintData.get(), dim * dim * 4);
  }
  