CesiumVectorStyle uses line style colorMode for polygon fill

[kring]

Instead of the polygon's own fill colorMode.

Looks like a copy/paste error here:
https://github.com/CesiumGS/cesium-unreal/blob/v2.18.0/Source/CesiumRuntime/Private/CesiumVectorStyle.cpp#L43

Originally reported here:
https://community.cesium.com/t/cesium-unreal-vector-overlay-branch-complie-error/42409/9