Fix warnings in public headers

[kring]

Somewhere along the way, we started identifying our public header files to CMake as SYSTEM. This is the right-ish thing to do for libraries that consume cesium-native via submodule / add_directory, so that a project stricter warnings won't flag a bunch of warnings in cesium-native headers. But it means that legitimate warnings in the header files, even when building cesium-native, are ignored. I noticed this because apparently some old versions of CMake don't treat SYSTEM headers specially, and the build had a whole bunch of warnings.

So we should:

• Tweak our CMake config so that the headers are SYSTEM outside cesium-native, but non-system within it.
• Fix the warnings

[lilleyse]

This also needs to be fixed so that clang-tidy can report errors for cesium-native's header files.

[kring]

@lilleyse have you already done some work on this? I've run into a couple of problems lately where cesium-native was building fine, but when we tried to use it Unreal there were new (legitimate) warnings that required us to go back and fix cesium-native. So I'd like to address this one, but don't want to duplicate work that you might have already done.

[lilleyse]

@kring I haven't done any work on this, feel free to take this one.

[azrogers]

I've introduced a solution for this in the clang-tidy PR #1004 using the PROJECT_IS_TOP_LEVEL CMake variable to decide whether headers should be marked SYSTEM or not. The PR fixes the resulting warnings for most headers, but the CesiumGltf headers still need to be addressed.