Let the user configure whether raster overlay textures are sRGB

[kring]

Currently, Cesium for Unreal marks all raster overlay textures as being sRGB. This is a big problem if the pixel values are meant to be some sort of data rather than an sRGB color.

Instead, we should make it a configurable property of the raster overlay. As indicated in this TODO:

cesium-unreal/Source/CesiumRuntime/Private/Cesium3DTileset.cpp

Lines 773 to 781 in 571c3f9

	auto texture = CesiumTextureUtility::loadTextureAnyThreadPart(
	CesiumTextureUtility::GltfImagePtr{&image},
	TextureAddress::TA_Clamp,
	TextureAddress::TA_Clamp,
	pOptions->filter,
	pOptions->group,
	pOptions->useMipmaps,
	true); // TODO: sRGB should probably be configurable on the raster
	// overlay