JPEG decoder slow on Firefox

Describe the bug Servers using the new JPEG encoding from RealVNC runs much slower than those using the Tight JPEG encoding. The issue only affects Firefox, not Chrome.

To reproduce Steps to reproduce the behavior:

1. Connect to VNC server using JPEG encoding
2. Do something that generates a lot of updates
3. Everything gets sluggish and memory usage spikes

Expected behavior Performance similar to Tight JPEG or better.

Client (please complete the following information):

• OS: Fedora 42
• Browser: Firefox
• Browser version: 142

Server (please complete the following information):

• noVNC version: 4cb5aa45ae559f8fa85fe2b424abbc6ef6d4c6f9
• VNC server: TigerVNC with RealVNC JPEG support
• WebSocket proxy: websockify

Additional context Issue reported to Firefox here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1986619