[scenes/opengl] Discard cached Lanczos textures for all toplevels
Summary:
Currently, the Lanczos filter doesn't discard cached textures for
internal and wayland clients.
Test Plan: Compiles.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D25616