Adopt SDWebImage's memory cache cost function, fix the nullable issue

[dreampiggy]

See SDWebImage/SDWebImage#2568

This PR add the memory cache cost function for custom image class SDFLAnimatedImage. Which is a wrapper for FLAnimatedImage. It store a internal frame buffer (called window) with a limit size.

[codecov]

Codecov Report

Merging #7 into master will decrease coverage by 6.59%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master       #7     +/-   ##
=========================================
- Coverage   77.77%   71.18%   -6.6%     
=========================================
  Files           2        2             
  Lines         108      118     +10     
=========================================
  Hits           84       84             
- Misses         24       34     +10

Flag	Coverage Δ
#iOS	71.18% <0%> (-6.6%)	⬇️

Impacted Files	Coverage Δ
.../Classes/FLAnimatedImageBridge/SDFLAnimatedImage.m	66.19% <0%> (-10.86%)	⬇️

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