GR: expose surface(...) display options, fix hardcoded constants

[t-bltg]

This PR add:

1. Expose the options described in GR surface's table;
2. Add the possibility to override hardcoded nx and ny when re-griding.

NOTES:

• name used: :display_option is arbitrary, maybe there is a better naming ?
• using series[:extra_kwargs] seems appropriate since this is specific to the GR backend
• should we document this or not ?

mwe

using Plots; gr()
import GR

main() = begin
  meshgrid(x, y) = (ones(eltype(y), length(y)) * x', y * ones(eltype(x), length(x))')

  x, y = meshgrid(-2:.25:2, -2:.25:2)
  f = x .* exp.(-x.^2 - y.^2)

  surface(x[:], y[:], f[:], display_option=GR.OPTION_SHADED_MESH, nx=50, ny=50, legend=:none, c=:viridis)
  return
end

main()

without PR

with PR

[codecov]

Codecov Report

Merging #3591 (094b9f8) into master (bba971f) will decrease coverage by 0.08%.
The diff coverage is 87.50%.

❗ Current head 094b9f8 differs from pull request most recent head e42b4ad. Consider uploading reports for the commit e42b4ad to get more accurate results

@@            Coverage Diff             @@
##           master    #3591      +/-   ##
==========================================
- Coverage   65.12%   65.03%   -0.09%     
==========================================
  Files          27       27              
  Lines        6660     6663       +3     
==========================================
- Hits         4337     4333       -4     
- Misses       2323     2330       +7     

Impacted Files	Coverage Δ
src/backends/gr.jl	88.65% <87.50%> (+0.03%)	⬆️
src/axes.jl	83.56% <0.00%> (-0.47%)	⬇️
src/recipes.jl	57.75% <0.00%> (-0.46%)	⬇️
src/layouts.jl	68.06% <0.00%> (-0.25%)	⬇️

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[BeastyBlacksmith]

Thats all fine and should definitely be documented in the GR backend section.