Vouch request: fix out-of-bounds Kitty placeholder source rects in libghostty-vt

[johnrichardrinehart]

What do you want to change?

I want to change the KiTTY graphics processing logic to avoid a SIGABRT.

Why do you want to make this change?

Because without this change, some copy-pastes crash my terminal (monstar).

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[jcollie]

!vouch

[johnrichardrinehart]

Thanks @jcollie !