steamwebhelper inconsistently crashes with multiblock enabled

This isn't a regression in FEX, but seemingly a change on the Steam side generating different code and causing multiblock to break.

Disabling multiblock seems to resolve the issue, but it's a bit inconsistent with how frequently it crashes.

Typical logs are
```
SP Desktop_uid0-'Steam': WARNING: cancelling DoClose() for bHideOnClose, but using Window.Close() is preferable
<...>
[2026-02-26 17:10:13] Browser - launching child process with: /proc/self/exe --type=renderer --crashpad-handler-pid=1696900 --enable-crash-reporter=, --change-stack-guard-on-fork=enable --enable-chrome-runtime --user-data-dir=/home/ryanh/steam_arm64/config/htmlcache --user-agent-product=Valve Steam Client --buildid=0 --steamid=0 --valve-initial-threadpool-size=188
[2026-02-26 17:10:13] Webhelper: An X Error occurred on display 0x7fff3410d0d0
[2026-02-26 17:10:13] X Error of failed request: BadDamage (invalid Damage parameter)
<...>
```
The X errors are a red herring as this is just a side-effect of chrome killing the process and SDL not understanding the window going away.

Bisecting this isn't possible, as it's not a FEX regression, it has instead been a lurking bug in FEX's multiblock for many months.
Trying to figure out exactly what we're breaking in multiblock is going to be hard.