Make sure mapping is not stateful

[thomasp85]

This PR fixes the "statefulness" of the mapping field introduced in #4265 by aligning it with how we now treat geom_params and stat_params. In fact the state has existed since the introduction of LayerSf where the mapping would be modified to ensure the geometry was catched.

This clean-up leaves a couple of dangling issues, mainly the fact that default_mapping no longer needs to be passed around. It is kept as-is so we don't change the signature of the guide calculation and break extension packages. We can remove it entirely when we rewrite the guide functionality to ggproto

[clauswilke]

Sorry if I'm dense. Could you explain in a few more sentences what exactly is being done?

Also, if default_mapping is kept just to maintain the signature, can you point that out in a comment and/or add a check that it's actually a missing argument? Or alternatively, replace with ...?

[thomasp85]

plot-level mappings and layer-level mappings needs to be fused together - earlier this was done all over the place and let to bugs which resulted in us moving the fusing into setup_layer(). This altered the state of the layer since it potentially modified the content of the mapping field. This was already being done in LayerSf where a geometry mapping was added if missing.

With this PR mapping is never altered, but the fused mapping is stored in a computed_mapping field similar to what is being done for computed_geom_params and computed_stat_params. This means that apart from computed_* fields layers remain stateless and are easier to reason about.

I already added a note at the guide_geom generic that default_mapping is unused and should be removed come rewrite

[clauswilke]

Ok, looks good to me.

I notice that there doesn't seem to be a unit test for the inherit.aes argument, so maybe add a very simple one.

[yutannihilation]

Looks good. It seems I was kind of aware about the need to treat the statefulness at the time of #4265 (comment), but couldn't come up with any solutions. Thanks for catching!