This is a sub issue of https://bugs.webkit.org/show_bug.cgi?id=231653

As Bug231653 pointed out, current blurring causes a mosaic artifacts when the blur radius is large.

The promissing solution is to shrink the texture before applying blur filter and expand again, which seems a common technique and skia uses it as well.
ref: https://github.com/google/skia/blob/0f9e50daa87997d376bf5fb60b06ab5b15c63ed9/src/gpu/ganesh/GrBlurUtils.cpp#L2580-L2586.

This is the main sub issue of Bug231653 but separeted from Bug231653 because But231653 will includes additional refactoring issue and performance issue.