fixes flutter/flutter#119974

This duplicates the math found in skia at
https://skia.googlesource.com/skia/+/cf2131f85d57f938472e50b6643c3c2348709a55/gm/blurrect.cpp#484

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…... (#47181)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/fuchsia-linux-sdk-flutter-engine
Please CC [email protected],[email protected],[email protected] on the revert to ensure that a human
is aware of the problem.

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Make sure to clear the buffers before assigning to the recycler, as the dtor was racing with the raster thread usage.