Make layouts tolerant of having too-few items

[BioTurboNick]

A big headache I have is that you currently have to have exactly the right number of layout spots for the number of plots you want to render. It makes rendering variable numbers of plots a bit of a headache.

This PR loosens the constraints so that fewer plots are acceptable, while too many plots still errors. One step towards a more flexible layout. that will continue with #4234

plot(plot(), plot(), plot(), layout = grid(2, 2))

[codecov]

Codecov Report

Merging #4247 (1c706e2) into master (ec77b97) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4247   +/-   ##
=======================================
  Coverage   77.87%   77.87%           
=======================================
  Files          28       28           
  Lines        7115     7111    -4     
=======================================
- Hits         5541     5538    -3     
+ Misses       1574     1573    -1     

Impacted Files	Coverage Δ
src/layouts.jl	75.81% <100.00%> (+0.29%)	⬆️
src/args.jl	74.31% <0.00%> (-0.16%)	⬇️
src/types.jl	82.60% <0.00%> (ø)
src/subplots.jl	95.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec77b97...1c706e2. Read the comment docs.

[BioTurboNick]

I see it passes all current tests, so I'll add one to enforce the permissive behavior if it otherwise checks out.