Write ids from webgpu shaders into the Ids attachment

- This commit adapts the shaders to write the cell, prop, composite and process id into the 4 channels of the Ids attachment.
- All id values are written as 32-bit unsigned integers.
- Merged `OverrideColorDescriptor` struct inside the `MeshAttributeDescriptor` to resolve padding issues which came up when the `CompositeID` and `ProcessID` members were added.