Vertex normals are needed for normals maps and therefore are packed and send
to the device alongside the other float3 attributes. However, we already pack
and send vertex normals through `DeviceScene.tri_vnormal`.
This removes the packing of vertex normals from the attributes buffer, and
reuses `tri_vnormal` in the kernel for normals lookup for normal maps, which
reduces memory usage a bit, and speeds up device updates.
This also fixes potential missing normals updates following rB12a06292af86,
since the need for vertex normals for normals maps was overlooked.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D12237
b8ecdbcd96