Fix clipped render when using geometry generator symbol in layout items

[nyalldawson]

Fixes #58909

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 82347f7)

[troopa81]

can we not setClipFeaturesToExtent to false for this very symbol, or set an empty extent before rendering instead of adding a new flag?

[nyalldawson]

we not setClipFeaturesToExtent to false for this very symbol,

That's not trivial either, we'd need to set it for all subsymbols for the symbol. The flag is less code 😁

or set an empty extent before rendering instead of adding a new flag?

I thought about that, but I'm concerned that it might cause a new regression if someone has symbology designed around the extent being available.