HasVertexPositionOffsetConnected returns true even if WorldPosition output isn't connect to output

FMaterialCachedExpressionData::UpdateForExpressions iterates on all expressions, even for those that are not enabled in a material instance or not even connected to output. This will enable features (such as WPO) that wouldn't be used since the connection may be on a disconnected path, and this can be create performance issues and make it difficult to debug.

This comment in IsGBufferLayoutSupportedForMaterial hints at the confusion: "FMaterialShaderParameters::bHasVertexPositionOffsetConnected means that the material could have WPO".

In other code, the word connected means something else, for example in U*Material::IsPropertyConnected,* it is considered as "is property used and different from the usual default", such as FMaterialResource::IsNonmetal (the connected constant may be a 1 on Metallic pin)

The issue is relevant for decals too: attributes writing is enabled if properties are connected (see ComputeDecalBlendDesc).

1. Create a new material. Create a SetMaterialAttributes node (leave it disconnected from material output) with WorldPositionOutput pin and connect to it a Time node.
2. UMaterial::HasVertexPositionOffsetConnected returns true, even if WorldPositionOutput is not connected to output.

Example Material:

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