fix: preserve MXCSR around earcut triangulation to support UE5 builds…

[Johnly1986]

… UE5 enables SSE DAZ (Denormals Are Zero) and FTZ (Flush To Zero) MXCSR flags on worker threads in packaged Development/Shipping builds. These flags cause mapbox::earcut's floating-point geometry predicates (signed area, point-in-triangle, etc.) to flush denormal intermediate values to zero, producing incorrect triangulation results for polygons with near-collinear vertices.

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[azrogers]

@Johnly1986 Thanks for the PR! Can you run down the author checklist in the template there? Particularly we need you to fill out the Contributor License Agreement - otherwise we can't take a look at your code!

[j9liu]

Hi @Johnly1986, can you make sure to fill out the Contributor License Agreement? We're not seeing your username in our database, which prevents us from giving this a review.

[Johnly1986]

Hi @j9liu , I've just signed the CLA — could you give it a recheck when you get a moment? Thanks! 🙏

[j9liu]

Confirmed that we received your CLA, thanks @Johnly1986! We'll take a look when we get the chance.

[kring]

@Johnly1986 since this is an Unreal-specific problem (mostly), would it be reasonable to change the mode at the start of each task in cesium-unreal and restore it at the end? So here:
https://github.com/CesiumGS/cesium-unreal/blob/main/Source/CesiumRuntime/Private/UnrealTaskProcessor.cpp

My thinking is:

1. Calling _mm_setcsr could have a bit of overhead, so doing it less often is good. Probably minor because triangulatePolygon is a pretty chunky call.
2. Perhaps other cesium-native computations are affected by this as well, we just haven't noticed yet.
3. It's better not to put Unreal-specific concerns in cesium-native if we have a good alternative.