Try to refactor the ImageManager trigger published value, avoiding the View will refreshing when deallocing

[dreampiggy]

This may solve #176

The callstack trace shows that UIHostingView's dealloc method will trigger the View's onDisappear method. And it use the unownded but not weak for the internal storage. If we trigger another body refreshing call from the Publisher (in the #176 is because of the ImageManager.isLoading property), this will cause the crash because the internal storage already been destroyed.

Instead, I try to avoid setting isLoading inside cancel. Tthis property is previouslly used to supports the delayPlaceholder feature.

delayPlaceholder in SDWebImage means:

• If the loading has never started, only show the indicator (if available) but not placeholder.
• If first loading failed, show the placeholder and disappear the indicator (if available)

So, instead I use the error to check whether there are loading failure. This can keep this function as well.

[dreampiggy]

Try using the demo code, function works as expected.

[codecov[bot]]

Codecov Report

Merging #180 (d175c0d) into master (cd8625b) will increase coverage by 0.20%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   74.81%   75.02%   +0.20%     
==========================================
  Files          11       11              
  Lines         969      965       -4     
==========================================
- Hits          725      724       -1     
+ Misses        244      241       -3     

Flag	Coverage Δ
ios	71.11% <50.00%> (+0.20%)	:arrow_up:
macos	75.77% <50.00%> (+0.23%)	:arrow_up:

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Impacted Files	Coverage Δ
SDWebImageSwiftUI/Classes/WebImage.swift	87.97% <20.00%> (-1.47%)	:arrow_down:
SDWebImageSwiftUI/Classes/ImageManager.swift	84.00% <100.00%> (+8.56%)	:arrow_up:

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