fix: force FBO recreation for Chromium/Electron windows after dock restore

[Mr-ZeLong]

Problem

When Chromium/Electron apps (QQ, WeChat, Chrome, Edge, Brave, etc.) are shown from the dock, Mutter's ClutterOffscreenEffect reuses the offscreen FBO populated during the unminimize animation. The FBO was populated at intermediate (scaled) resolutions with whatever stale Wayland buffer the client still holds, resulting in blurry window content that persists until the mouse enters the window.

This is reproducible on GNOME 50 with Dash to Dock, 100% integer scaling, XWayland and native Wayland clients.

Root Cause

1. Mutter's ClutterOffscreenEffect.pre_paint() determines the offscreen FBO size from the actor's paint_volume during the unminimize scale-up animation. The FBO is populated each frame at progressively larger sizes.
2. Chromium/Electron apps may not deliver fresh Wayland buffers immediately after unminimize, so Mutter renders stale/low-res content to the FBO.
3. After the animation ends, the FBO size check passes, so the FBO is reused indefinitely with the stale content.
4. queue_repaint() only re-runs the GLSL shader on the existing FBO — it does not force Mutter to re-render the window content into the FBO.
5. Mouse enter triggers wl_pointer.enter → Chromium delivers a fresh frame → FBO content becomes sharp.

Fix

Force FBO invalidation by toggling the effect's enabled state (enabled=false destroys the offscreen, enabled=true + queue_repaint allocates a fresh FBO with the current window buffer).

Two trigger paths:

• onUnminimize: 250ms delay for Electron's Wayland renderer to deliver a settled frame, then toggle + refresh.
• onFocusChanged: When window is behind others and brought to front from dock.

Also adds periodic shader refreshes for Chromium windows on fractional-scaled displays.

Non-Chromium windows are completely unaffected — updateEffectWithFboInvalidate skips the toggle for them.

[flexagoon]

@Mr-ZeLong did you read the pull request guidelines?