Implement 2D GPU flux-differencing volume kernels

The 3D GPU volume kernels have been developing and we are waiting to have a stable implementation in the main. Once this is done, 2D GPU kernels should be updated accordingly.

Wait for the 3D GPU volume kernels to reach a stable implementation in the main branch, then use them as the reference for the existing 2D GPU kernels. Compare the 2D and 3D implementations and update the 2D kernels accordingly; done means the 2D GPU flux-differencing volume kernels match the stable design.

Written by the indexing model from the issue text.