Compute spacing/width separately for unshared axes

[mwaskom]

Motivating example (from #3112):

(
    sns.load_dataset("penguins")
    .melt(["species"], ["bill_depth_mm", "bill_length_mm"])
    .pipe(so.Plot, x="value", color="species")
    .facet("variable")
    .add(so.Bars(), so.Hist(common_bins=["col"]))
    .share(x=False)
)

Before:

After:

(This is a little artificial because you could just use Plot.pair here but there are other cases where you'd actually want faceting).

[codecov]

Codecov Report

Merging #3119 (f9a4c96) into master (021a20f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3119   +/-   ##
=======================================
  Coverage   98.41%   98.41%           
=======================================
  Files          76       76           
  Lines       24056    24076   +20     
=======================================
+ Hits        23675    23695   +20     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/utils.py	95.46% <ø> (ø)
seaborn/_core/plot.py	99.33% <100.00%> (+<0.01%)	⬆️
tests/_core/test_plot.py	98.68% <100.00%> (+0.01%)	⬆️