Fix editor crash when unsupported Resource is dropped in scene

[luevano]

Follow up on #89106

Change CanvasItemEditorViewport::can_drop_data to check if files are valid Resource types (PackedScene or Texture2D) instead of checking for extension types (where .tres is valid for Texture2D but it's not an exclusive extension for it).

Change safeguard check from #89106 to an error message as suggested. Though, from my testing I can't seem to trigger this error message as the Resource type check handles this, too.

Changed nested if statements to exit immediately to avoid unnecessary code nesting.

Also fixes #89093, so it actually supersedes #89106

[akien-mga]

CC @ryevdokimov

[luevano]

I also fixed an issue that when dropping a scene into itself (cyclic dependency) would create an undo-redo history action and display "Create Node" in the godot diagnostics window; I found it while testing this and it's shown here: #89106 (comment) (in my old PR that is now superseded by this PR). It now behaves like this:

godot.windows.editor.x86_64_0s6IjBqK0L.mp4

The only downside is that there is no easy way to display the "cyclic dependency" error that I know of (that doesn't pollute the logs). Can I include this change in this PR, too? It's related.

[ryevdokimov]

Looks good.

2024-03-04.08-24-20.mp4

[luevano]

Any comment on my last message @ryevdokimov / @KoBeWi ? I haven't included the changes to avoid dropping cyclic dependencies as I found a fix after the fact

[KoBeWi]

I think that fix should go to separate PR, unless it's closely related.

[luevano]

It's a change on the same can_drop_data and minor one in the _cyclical_dependency_exists to mark it as const.

[KoBeWi]

I think that other change should be in _perform_drop_data(), to prevent creating undo actions if no node was created. Thus it's better as a separate fix.

[luevano]

Edit: sorry, didn't see your latest comment. I did try doing it there, but I failed. Will look into it with more time later.

To keep this simple and prioritize the crash fix, I'll leave it as is. Once this is merged I can create a PR for the change I'm describing

[akien-mga]

Thanks!