Fix crash caused by glyph splitting

[arnaud-secondlayer]

Fix a crash caused by glyph splitting ( #928 ), with a test that demonstrates the problem.
Glyph splitting assigns distinct glyphs to the same index in the original text, we need to store previously used indices to make sure we do not re-use the same index while overwriting span glyphs.

[LaurenzV]

@RazrFalcon Does that make sense to you, in case you still remember? Unfortunately I'm not at all familiar with that part of the code.

[RazrFalcon]

Yeah, it's hard to say. I don't know how this part should be implemented myself. But overall it should be fine.

My only suggestion would be to move HashSet out of the loop and clear it instead of creating a new one each iteration.

And put this comment directly in the code:

Glyph splitting assigns distinct glyphs to the same index in the original text, we need to store previously used indices to make sure we do not re-use the same index while overwriting span glyphs.

Otherwise, tests are passing and that's good.

[LaurenzV]

Thanks!