Reduce bezel redraw work and release CGColors

[emanuelst]

Caches the bezel background until its size, opacity, or color changes, and releases temporary CGColor objects after layer assignment. No behavior or appearance changes are intended.

AI-assisted; reviewed and tested locally.

[TermiT]

Thanks for this — the background caching is correct, MRC-safe, and merges cleanly.

One request before merging: while you're in BezelWindow.m, could you also fix the CGColor leaks in -update? The CGColorCreateGenericRGB(...) results used for the background/border there (around lines 136–138, plus the similar ones in init) are never CFRelease'd, so they leak on every window update. Since -update runs on every bezel refresh, CFRelease-ing those is arguably a bigger per-update win than the pattern-image cache itself, and it fits naturally alongside this change.

Minor note on framing: the bezel is orderOut'd (hidden) while idle, so this reduces redraw cost while the bezel is on screen rather than idle CPU. The constant idle CPU in #282 is the ~1s pasteboard-polling timer, so I'd like to keep #282 open for that separately. But this is a solid improvement for active-bezel redraws — happy to merge once the CGColor releases are folded in.

[emanuelst]

Thank you for the feedback! I added the CGColor releases.