Find extrema when creating an AstroImage

[giordano]

This would make render much faster.

My only concern is that I'm not yet sure render would be used very often

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff          @@
##           master   #13   +/-   ##
====================================
  Coverage      95%   95%           
====================================
  Files           3     3           
  Lines          20    20           
====================================
  Hits           19    19           
  Misses          1     1

Impacted Files	Coverage Δ
src/AstroImages.jl	100% <100%> (ø)	⬆️

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[aquatiko]

@giordano render is used very often to render the canvas in AstroImageView.jl. I think it will be good to include this now.

[giordano]

Would you be able to merge master on that branch and fix conflicting changes? You can open a PR on https://github.com/giordano/AstroImages.jl against branch minmax