Implement clipboard_get/has_image for X11

[Setadokalo]

Following up on the work done in #63826, this implements clipboard_has_image() and clipboard_get_image() on the X11 backend. This copies a lot of structure from the existing clipboard_get implementation.

This also only implements png-based clipboard transfers, but that seems to be how the windows implementation works too. Other file types shouldn't be too hard to implement, but every app I've tested with supports png target, even when the original file is a jpg or some other image type.

This is my first time working with X11 APIs, so testing from other linux users would be greatly appreciated!

[Setadokalo]

With further testing, it seems particularly large images do trigger the incremental transfer mechanism (which I didn't experience in testing), and the incremental transfer does not work reliably. I'm looking into it now.

[Setadokalo]

Large file transfers should now be fixed, but after fixing it I'm now almost certain the incremental handling for text clipboard data is and has been broken. Has it been tested? Unless someone asks me to, I won't fix it in this PR to keep the scope from creeping.
Testing the incremental transfer of text and it seems to work fine. Not sure how, but hey, as long as it works.

[akien-mga]

Style/code quality review. Looks great overall, at least as far as X11 API usage can look ;)

[YuriSizov]

@Setadokalo Please let us know if you'll be able to address the review comments any time soon! Maintainers can finish it on your behalf if you are not available or don't respond.

[akien-mga]

Could you squash the commits? See PR workflow for instructions.

[Setadokalo]

I'm currently rewriting the last part of that function as I think you're right about it potentially double-freeing valid_targets. I'll squash it after I make sure it still works.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉