Refactor model pipeline factoring in OIT pass

In the glTF edge visibility PR, there are cases where shader output declarations conflict with OIT’s output layout, even if the code paths are not executed in the OIT pass.

I introduced a temporary solution:
A new flag CESIUM_REDIRECTED_COLOR_OUTPUT was introduced to indicate that a shader has been derived/modified by Cesium’s rendering pipeline. This allows MRT features and other passes to conditionally declare output variables only when not conflicting with the redirected shader outputs.

However, we should consider a broader refactor of the model pipeline to handle this issue in a more systematic way.

https://github.com/CesiumGS/cesium/pull/12859