Allocates a fresh temporary UIMesh sized for vertices and indices, with slices for the requested extras .
Channels not requested return as empty slices on the resulting UIMesh. Use the
result as input to DrawData.SetMesh for tessellation refinement or wholesale geometry
replacement. Memory is valid for the duration of this callback and any jobs added via
MeshModificationContext.AddMeshModificationJob.
Requested channels that are NOT enabled on the panel's ExtraVertexChannels mask
are dropped and the resulting UIMesh returns empty slices for those channels. Check
slice lengths before filling.