Fix Gaussian Blur's bug which take half of the blur radius compared to the standard, should match Core Image's behavior

[dreampiggy]

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This merge request fixes / refers to the following issues: ...

Pull Request Description

See W3C Standard: https://www.w3.org/TR/SVG11/filters.html#feGaussianBlurElement

The gaussian blur should use 3 times box convolve, which the box size should be equal to the following fomula:

let d = floor(s * 3*sqrt(2*pi)/4 + 0.5)

But however, currrent we calculate with this fomula instead, which is wrong:

let d = floor((s * 3*sqrt(2*pi)/4 + 0.5) / 2)

• Wrong One: React Native iOS: https://github.com/facebook/react-native/blob/master/Libraries/Image/RCTImageBlurUtils.m#L48
• Correct One: Google Chrome for iOS: https://chromium.googlesource.com/chromium/src.git/+/81.0.4028.1/ios/chrome/browser/ui/util/uikit_ui_util.mm#363

[dreampiggy]

Sample image:

Blur Radius: 3 (which is current behavior when use sd_blurredImageWithRadius with radius 6, which is wrong)

Blur Radius: 6 (which is the actually radius 6)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7e3482d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2927   +/-   ##
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  Coverage          ?   84.04%           
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  Files             ?       65           
  Lines             ?     6404           
  Branches          ?        0           
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  Hits              ?     5382           
  Misses            ?     1022           
  Partials          ?        0

Flag	Coverage Δ
#ios	84% <100%> (?)
#tvos	83.71% <100%> (?)

Impacted Files	Coverage Δ
SDWebImage/Core/SDAnimatedImage.m	86.87% <ø> (ø)
SDWebImage/Core/UIImage+Transform.m	80.67% <100%> (ø)

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