BlurCache: don't rebuild the cache when dirtyRegion is empty

[xxmathias]

Problem

When KWin paints a blurred window with damage that doesn't intersect backgroundRect (e.g. shadow-only damage on focus changes, or repaint slop from effects like wobbly windows), the dirtyRegion is empty: nothing behind the window was repainted this frame, so there are no fresh pixels to compare against and prepareCache() bails before setting up the conditional render.

Previously this caused an ungated re-blur: all blur passes ran and drawToCache() overwrote the cached texture — the only place the cache was rewritten outside the conditional-render gate. Besides wasting GPU time on every such paint, this rebuilt the cache from the reference blit alone (which can contain never-initialized regions) and re-applied the noise and rounded-corner passes to it.

This is suspected to contribute to blur showing a stale "cached old state" instead of tracking live content behind the window.

Fix

Track this case as useCachedOnly in the paint data and skip the scene blit, all blur passes and the cache writes, only drawing the existing cached texture.

If no cache entry exists yet and there is no fresh data, bail entirely — there is nothing valid to build an entry from, and the on-screen draw is clipped to the deviceRegion anyway.

Notes

• colorMatrix/modulation declarations were hoisted above the new gate since drawCached() needs modulation outside it.
• Follows the existing "indent intentional for KWin diff" style for the added braces to keep the diff against upstream kwin minimal.

🤖 Generated with Claude Code

[xxmathias]

Rebased this onto your BlurCache rework. The merge conflicts are gone now.

I kept the useCachedOnly guard since it still seems to matter even with the new isFlushing flow: when the timer elapses you flush even if accumulatedDirtyRegion is empty, so a flush frame can land with an empty dirtyRegion (shadow-only damage on focus change). Without the guard that case blits + rebuilds the cache from stale data. So it ends up complementing the rate limiting rather than overlapping with it.

No rush, just wanted to unblock it since it had conflicts. Happy to drop it if you'd rather fold it into the rework yourself.