Decouple OmniTileset and OmniRasterOverlay

[lilleyse]

In supporting custom ellipsoids we introduced some coupling between OmniTileset and OmniRasterOverlay.

cesium-omniverse/src/core/src/OmniRasterOverlay.cpp

Lines 137 to 154 in e9a4bd0

	const CesiumGeospatial::Ellipsoid* OmniRasterOverlay::getEllipsoid() const {
	std::optional<const CesiumGeospatial::Ellipsoid*> maybeEllipsoid;
	const auto& tilesets = _pContext->getAssetRegistry().getTilesets();
	for (const auto& pTileset : tilesets) {
	if (CppUtil::contains(pTileset->getRasterOverlayPaths(), _path)) {
	const auto pEllipsoid = pTileset->getEllipsoid();
	if (!maybeEllipsoid) {
	maybeEllipsoid = pEllipsoid;
	} else if (*maybeEllipsoid != pEllipsoid) {
	// If not all tilesets that reference this raster overlay use the same ellipsoid then set it to nullptr
	return nullptr;
	}
	}
	}
	return maybeEllipsoid.value_or(nullptr);
	}

If you attach a raster overlay to tilesets with different ellipsoids it will give up and use the default ellipsoid.

I think a better approach would be to create an OmniRasterOverlayInstance object that points to the OmniRasterOverlay and owns the Cesium Native object. That way each instance can receive its own ellipsoid.