High Virtual Texture Sample/Lookup Cost when using a Virtual Texture in a Material Layer Blend

Some Material Attribute blends in Material Layer Blends can cause Virtual Texture samples to be duplicated in HLSL. This can be avoided by sampling the Virtual Texture in the Parent Material, and passing it through the layers into the blend.

1. Download the attached test project and open
2. Open the Material Instance "/Game/MI_Expensive_Blend"
3. Observe that its "VT Lookups (Est.)" is 13
4. Observe that the HLSL will call `ProcessMaterialVirtualColorTextureLookup(...)` many times in `CalcPixelMaterialInputs(...)`. (around lines 4194, 4248)
5. Open the Material Instance "/Game/MI_Cheap_Blend"
6. Observe that its "VT Lookups (Est.)" is 3
7. Observe that the HLSL will call `ProcessMaterialVirtualColorTextureLookup(...)` once in `CalcPixelMaterialInputs(...)` and use the output for all blends. (around lines 4194)
8. Open the two Material Layer Blends "/Game/SupportAssets/MLB_VT_Cheap" and "/Game/SupportAssets/MLB_VT_Expensive".
9. Observe the difference in where they fetch the VT mask. The Expensive blend fetches the mask in a parameter in the blend itself, while the Cheap blend fetches the mask from the Parent Material.