This change makes it so baking displacement to the high-resolution mesh
(Use Low Resolution Mesh option is OFF, displacement is calculated
between top multi-resolution level and subdivided viewport level mesh)
uses texture UVs and tangent space from the high-res mesh.
This matches intended use-case when object baked with such configuration
have subdivision surface applied to them bringing overall resolution to
the same level as the highest multi-resolution level.
The issue was simple to see when baking high-res displacement for an
object which uses "Keep Corners, Junctions" UV smoothing.
The unfortunate aspect is increased memory usage due to calculation of
the grid index and grid UV mapping, but it is not too bad (12 bytes per
loop, so is like 48Mb per million face). Feels like there is a way to
optimize it by utilizing knowledge that high-res mesh faces are created
in a specific order, but also feels it is not that important at this
moment.
Pull Request: https://projects.blender.org/blender/blender/pulls/144935
8348438cae