Don't allow copying file into its own path

[KoBeWi]

When you copy file into its source path, it will truncate it and then fail to write, resulting in empty file. This PR adds a check to prevent that.

[AThousandShips]

I'd suggest a more broad improvement here:

• Offload copying inside the file system proper to platforms, using more efficient low level methods there
• Use this just for copying between access types, like res:// -> user://, etc.

[KoBeWi]

Offload copying inside the file system proper to platforms, using more efficient low level methods there

Like which one? The only difference in the OS-specific methods is that they always operate on absolute paths. And copy() is not even system-specific, it has general implementation. I'm not sure how to solve it more reliably, other than writing a method that untangles absolute paths.

Use this just for copying between access types, like res:// -> user://, etc.

??

[AThousandShips]

I mean that this method is naïve and inefficient, and that unless we can't use a native method we might be better off using a native one, only if we're copying from inside a pck or exported project, or from memory, or network, it'd be useful to use native methods

But it does use absolute paths though, underlyingly, just in FileAccess, otherwise they couldn't do anything either, so id say that the solution here is both to improve performance, and solve this issue properly, by letting the platform handle it

[KoBeWi]

The performance is not a concern here and I don't know any native method to handle that.

[AThousandShips]

There should be generally, absolute paths aren't an issue as such here, or we couldn't do any file management, but anyway, I don't see this very specific check as a very useful safeguard generally

[akien-mga]

If this prevents truncating files, I'd say it's a good improvement already, even if the UX or getting an error message like this isn't great.

[akien-mga]

Thanks!