Mark these copy constructors as 'explicit' in `blender::animrig`:
- `Slot`
- `StripKeyframeData`
- `ChannelBag`
The copy constructors for the other related classes were already
`explicit` or `deleted`.
This prevents bugs by disallowing implicit copies. For example:
```cpp
ChannelBag cbag = agrp->channel_bag->wrap();
```
This should have been a reference (`ChannelBag &cbag`), an easy mistake
which is now caught by the compiler (and fixed in this commit).
No functional changes. The implicit copy that was removed was just
inefficient, but didn't produce the wrong results.
Pull Request: https://projects.blender.org/blender/blender/pulls/128424
38f5b8d7d4