fix(call): improve virtual background performance

[Antreesy]

☑️ Resolves

Goal is to reduce GPU load while maintaining similar blur/mask quality and responsiveness

fix(virtual-bg): use requestVideoFrameCallback when available

• render loop was scheduled via TimerWorker that fired with 1000/frameRate interval
• this allowed delay for up to 1-2 frames
• requestVideoFrameCallback fires exactly when a new video frame is available
• cancel on stopping the effect
• add _lastInferenceTimestamp guard for MediaPipe's VIDEO mode (strictly requires incrementing values)
• should reduce delay, but negligible

fix(virtual-bg): keep _lastMask open while being reused

• when inference hasn't finished yet, previous _lastMask is reused for frame output
• _lastMask references should be kept open until replaced ot not needed anymore
• most noticeable, if inference is throttled (manually or due to perf issues)

fix(virtual-bg): avoid GPU/canvas reallocation every frame

• texMaskFiltered, texBlurred1, texBlurred2 were reallocated on every single render() call
• reallocating GPU texture every frame is a confirmed overhead on some GPUs/drivers
• needs to be only reallocated when the actual size changes
• similar with output canvas element
• impact is negligible, so more a hardening

AI (if applicable)

• The content of this PR was partly or fully generated using AI

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

Tested on Windows with Intel(R) UHD Graphincs

🏚️ Before	🏡 After
A	B

🚧 Tasks

• Check for possible mask freezing in _runInference with active video frame and _postProcessing
• Uncaught (in promise) RuntimeError: divide by zero
• Other errors

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required