Substrate- black outline appears when using sharp contrast mask to blend slabs

Substrate materials have a thin border of black pixels between slabs.

These lines can be made more apparent by changing the Diffuse Albedo color of the two slabs to pure white. This will remove the visual gradient on the horizontal blend case.

This might be the result of information in the Substrate.Material buffer being preserved from previous frames. The black border fades quickly when the view is not changing, but remains if TAA/TSR is enabled, due to the sub-pixel jitter.

In a GPU Dump, we observed that some slices of the Substrate.Material buffer contained garbage data as well as relevant pixels. Most of the slices were not cleared in the GBuffer clear pass. Some slices were not bound as render targets, others contained non-black pixels after the pass.

Also tested on //UE5/Release-5.5, CL: 36784631

1. Download the Licensee's test project and open (see addl info URL)
2. Observe the single-pixel black lines at the edge of the gradients for the Vertical and Horizontal blend samples that do not use parameter blending.
3. Observe that as you move the camera closer and further from the meshes, the black lines stay the same size relative to the screen.
4. Disable Anti-Aliasing.
5. Observe that the single-pixel black lines fade and vanish very quickly when not moving.
6. Set Anti-Aliasing to TSR
6. Observe that the black lines fade, but never completely vanish when not moving.