Fix blur noise effect using incorrect scaling value

Summary:
`QGuiApplication::primaryScreen()->logicalDotsPerInch()` sometimes reports incorrect scale value, for example 95.951 (or even lower) instead of 96.0 at 1x scale and this caused to simply darken blurred areas instead of noising them.
This patch sets a minimum 1x scale for the noise texture, because realistically we will never be below that.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12519