message: change apply rtc center to use cesium-rtc extension
diff: #include "CesiumGeometry/AxisTransforms.h"
  #include "CesiumGeometry/Rectangle.h"
  #include "CesiumGltf/AccessorView.h"
  +#include "CesiumGltf/ExtensionCesiumRTC.h"
  #include "CesiumGltf/ExtensionMeshPrimitiveExtFeatureMetadata.h"
  #include "CesiumGltf/ExtensionModelExtFeatureMetadata.h"
  #include "CesiumGltf/TextureInfo.h"
  @@ -1305,27 +1306,24 @@ namespace {
  void applyRtcCenter(
  const CesiumGltf::Model& model,
  glm::dmat4x4& rootTransform) {
  - auto rtcCenterIt = model.extras.find("RTC_CENTER");
  - if (rtcCenterIt == model.extras.end()) {
  - return;
  - }
  - const CesiumUtility::JsonValue& rtcCenter = rtcCenterIt->second;
  - const std::vector<CesiumUtility::JsonValue>* pArray =
  - std::get_if<CesiumUtility::JsonValue::Array>(&rtcCenter.value);
  - if (!pArray) {
  +
  + const ExtensionCesiumRTC* cesiumRTC =
  + model.getExtension<ExtensionCesiumRTC>();
  + if (cesiumRTC == nullptr) {
  return;
  }
  - if (pArray->size() != 3) {
  + const std::vector<double>& rtcCenter = cesiumRTC->center;
  + if (rtcCenter.size() != 3) {
  UE_LOG(
  LogCesium,
  Warning,
  TEXT("The RTC_CENTER must have a size of 3, but has {}"),
  - pArray->size());
  + rtcCenter.size());
  return;
  }
  - const double x = (*pArray)[0].getSafeNumberOrDefault(0.0);
  - const double y = (*pArray)[1].getSafeNumberOrDefault(0.0);
  - const double z = (*pArray)[2].getSafeNumberOrDefault(0.0);
  + const double x = rtcCenter[0];
  + const double y = rtcCenter[1];
  + const double z = rtcCenter[2];
  const glm::dmat4x4 rtcTransform(
  glm::dvec4(1.0, 0.0, 0.0, 0.0),
  glm::dvec4(0.0, 1.0, 0.0, 0.0),
  