clip on the rectangle containing the GridView instead of the view itself

Summary:
clip on the rectangle containing the GridView instead of the view itself

this avoid cutting the shadow of the fist line before any scroll on the view

Test Plan: shadow now looks fine

Reviewers: Elisa, januz, astippich

Reviewed By: Elisa, astippich

Maniphest Tasks: T8345

Differential Revision: https://phabricator.kde.org/D11761