Prevent an infinite loop in the contrast GUI

[jwahlstrand]

Convert new contrast limit values to the right type and only update if the new values are different from the old values.

Prevents a crash when the contrast bars cross.

Fixes #308

It would be ideal to add a test too, but I don't have time this morning.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.29%. Comparing base (e54547a) to head (d836781).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/contrast_gui.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   86.04%   85.29%   -0.76%     
==========================================
  Files           6        6              
  Lines         860      809      -51     
==========================================
- Hits          740      690      -50     
+ Misses        120      119       -1     

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