BlurEffect: drop render data on resume from sleep

[xxmathias]

Problem

After resuming from suspend/hibernate, the effect can wedge into a state where it keeps drawing stale cached content and never updates again. Observed on a ThinkPad T14 (amdgpu, Plasma 6.7.2, Wayland, BlurDecorations=true): after resume, window decorations stayed frozen at their pre-sleep pixels — titlebar buttons were visible but did not respond to clicks, because the on-screen titlebar no longer matched the live decoration. Unloading and reloading the effect via D-Bus cleared it, which pointed at stale per-window render state rather than config or KWin.

Cause

GPU memory is powered down across suspend (and fully lost across hibernate/S4), so the cached blur textures and the TextureComparer's counter SSBOs / query objects can come back holding garbage. A query stuck at "no pixels changed" permanently wedges the conditional render and dirty-region tracking: every paint takes the cached-only path and the stale texture is drawn forever. Nothing currently invalidates on resume — only viewRemoved/screenRemoved are handled.

Fix

Subscribe to logind's PrepareForSleep on the system bus and, on wake, drop every window's BlurRenderData — dual-Kawase textures, framebuffers, BlurCacheLRU entry, and the compare WindowData — so everything is rebuilt from scratch on the next paint. Also adds the previously missing explicit Qt6::DBus linkage (it only linked because kwin already loads libQt6DBus).

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