Fix vtt sprite generation ( issue #1033 )

[bnkai]

Fixes #1033

[WithoutPants]

This doesn't appear to work on Windows. I've set the generated directory to be tmp\[generated}. The escaped glob pattern looks like: tmp\\[generated\}\tmp\thumbnail_b7f0f0ee07f9f6d3_*.jpg, and the glob finds no files, despite the sprite files being present in that directory.

[bnkai]

@WithoutPants thanks for the feedback, i thought glob would first replace the escaped parts and then continue on windows but it seems that it just doesn't escape in windows

Note that for systems where the path separator is '\\', escaping is
disabled

I'll proceed with replacing doublestar glob with a custom function