[3.x] Verify GLTF indices to prevent crash with corrupt files

[lawnjelly]

Also verify prior to vertex optimization.

Fixes #94696

Error reporting is now:

Godot Engine v3.6.rc.custom_build (c) 2007-2022 Juan Linietsky, Ariel Manzur & Godot Contributors.
 ./modules/gltf/gltf_document.cpp:2949 - Condition "!Geometry::verify_indices(inds.ptr(), inds.size(), num_verts)" is true. Returned: ERR_FILE_CORRUPT
 ./modules/gltf/packed_scene_gltf.cpp:76 - Condition "err != Error::OK" is true. Returned: nullptr
 ./editor/editor_file_system.cpp:1764 - Error importing 'res://gltf/03_level_RocksMaze.gltf'.

Notes

• Could possibly produce an error dialog or something to alert the user, but erring on the side of caution here as that could be an annoyance if multiple files / multiple attempts at import, and we don't want to introduce annoyances immediately prior to 3.6 stable.
• Also now checks during vertex cache optimization, as the MRP did also crash within vertex optimization, and obviously the third party vertex optimization code isn't fault tolerant. Now it just returns false and flags an ERR_FAIL prior to attempting the optimization.

Other similar vulnerabilities

Although the linked bug (#94696) in GLTF import is closed by this PR, it is possible that a similar crash can still occur if add_surface_from_arrays() is called with invalid data from elsewhere in the source code.

We could in the long term decide to play it safe and verify all input indices during add_surface_from_arrays() (instead of during the GLTF import), however I'm not yet clear whether this could cause performance impact unnecessarily, so it may be worth waiting to see if we have any reports of similar crashes from elsewhere (I haven't seen any recently on github, and this vulnerability is not new, it has probably existed for years).

It would be very simple to move the verification check to add_surface_from_arrays() in a follow up PR if we later decide that would be better approach.

[clayjohn]

Looks good to me. As far as I can tell, in 4.x, we only validate indices at import time. I think it is fine to only add the check in places that are impacted by the specific bug report.

[lawnjelly]

Thanks!