Fix raster overlay undefined behavior

[kring]

This is a PR into #1552 so merge that first.
This depends on CesiumGS/cesium-native#1003 so merge that first, too.

Fixes #1551

[azrogers]

Trying to test this and CesiumGS/cesium-native#1003 out and I'm getting a lot of errors relating to the changes in main, particularly over gsl::span:

1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C2653: 'gsl': is not a class or namespace name
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C7568: argument list missing after assumed function template 'span'
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C2144: syntax error: 'unknown-type' should be preceded by ')'
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C2144: syntax error: 'unknown-type' should be preceded by ';'
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C2059: syntax error: ')'
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(29): error C2433: 'UnrealAssetAccessor::contentPayload': 'virtual' not permitted on data declarations
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(24): warning C4263: 'CesiumAsync::Future<std::shared_ptr<CesiumAsync::IAssetRequest>> UnrealAssetAccessor::request(const CesiumAsync::AsyncSystem &,const std::string &,const std::string &,const std::vector<CesiumAsync::IAssetAccessor::THeader,std::allocator<CesiumAsync::IAssetAccessor::THeader>> &,const int)': member function does not override any base class virtual member function
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\CesiumRuntime\Public\UnrealAssetAccessor.h(41): warning C4264: 'CesiumAsync::Future<std::shared_ptr<CesiumAsync::IAssetRequest>> CesiumAsync::IAssetAccessor::request(const CesiumAsync::AsyncSystem &,const std::string &,const std::string &,const std::vector<CesiumAsync::IAssetAccessor::THeader,std::allocator<CesiumAsync::IAssetAccessor::THeader>> &,const std::span<const std::byte,18446744073709551615> &)': no override available for virtual member function from base 'CesiumAsync::IAssetAccessor'; function is hidden
1>C:\Dev\prs\cesium-native-1003\Plugins\cesium-unreal\Source\ThirdParty\include\CesiumAsync\IAssetAccessor.h(57): note: see declaration of 'CesiumAsync::IAssetAccessor::request'

Does this PR then also depend on #1539? It seemed to be non-trivial getting the ue55 branch merged into this one to test it out, so I didn't go that far.

[kring]

Yeah, after the C++20 upgrade PRs were merged into cesium-native main, and main was merged into CesiumGS/cesium-native#1003, this became dependent on #1539. I merged that branch into here and changed the base branch accordingly.

[azrogers]

Looks great! Thanks @kring!