Throw when crop rectangle exceeds source bounds.

[JimBobSquarePants]

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Description

We should throw when a crop rectangle exceeds the image bounds. This helps prevent users misappropriating the method and expecting unintended results. Fixes #605

[codecov]

Codecov Report

Merging #726 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
+ Coverage   88.71%   88.71%   +<.01%     
==========================================
  Files         892      892              
  Lines       39752    39756       +4     
  Branches     2741     2741              
==========================================
+ Hits        35267    35271       +4     
  Misses       3786     3786              
  Partials      699      699

Impacted Files	Coverage Δ
...Tests/Processing/Processors/Transforms/CropTest.cs	100% <ø> (ø)	⬆️
.../Processing/Processors/Transforms/CropProcessor.cs	100% <100%> (ø)	⬆️
src/ImageSharp/Processing/CropExtensions.cs	100% <100%> (ø)	⬆️
...ImageSharp.Tests/Processing/Transforms/CropTest.cs	100% <100%> (ø)	⬆️
...ageSharp.Tests/BaseImageOperationsExtensionTest.cs	100% <100%> (ø)	⬆️
tests/ImageSharp.Tests/ImageOperationTests.cs	97.36% <100%> (-0.07%)	⬇️
...sing/Processors/Transforms/EntropyCropProcessor.cs	87.5% <100%> (ø)	⬆️

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