[Bug] Correct import relies on generator string being in form 'Tilt Brush vX.Y...'

[mikeskydev]

When importing a glTF from Open Brush into either the Tilt Brush or the Open Brush toolkit, it takes the unusual step of parsing the "generator" field of the glTF asset to see if it's from Tilt Brush, and depending on which version of Tilt Brush it applies specific modifications:
https://github.com/icosa-gallery/open-brush-toolkit/blob/v23.0.1/UnitySDK/Assets/TiltBrush/Scripts/Gltf/ImportGltf.cs#L362

This means that any glTF that doesn't match the generator regex above fails to import correctly, therefore Open Brush exports (and likely any exports from other forks!) have always been incorrectly shown in the toolkits.

Open Brush will be fixing this by hardcoding the generator string at the last version of Tilt Brush, but we should fix this for all users of the Open Brush toolkit going forward.