Reduce mask blur kernel size to 2.5 sigmas

This more closely matches the old behavior of PIL's Gaussian blur, and
fixes breakage when tiling.

See https://github.com/Coyote-A/ultimate-upscale-for-automatic1111/issues/111#issuecomment-1663504109

Thanks to Алексей Трофимов and eunnone for reporting the issue.