fix(text): fix substract with overflow panic

[Valentin271]

Fixes #197

This change avoids the panic, visually the behavior is the same.
However I'm not sure this is correct, because it means sometimes it won't be subtracted (and we get a wrong index?).

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I don't get why we would need to subtract one to the index of the glyph under the cursor only if the cursor is associated with the previous frame. Also removing this if Affinity::Before altogether doesn't seem to change anything ...
Have I misunderstood something in my reasoning?

[CosmicHorrorDev]

I believe this is the correct behavior. The affinity stuff is used to determine which run the cursor should be a part of when it's between two text runs. Subtracting one (kinda shaky logic that it's exactly one) ensures that the cursor falls on the previous text run when between two, and the only value that saturates would be zero which should only happen when you're already on the first text run

[Valentin271]

Ok I think I'm misinterpreting what a text run is. With your explanation it seems more like a chunk of text (and now it makes sense to be between two elements, selecting the previous glyph) whereas I understood it as kind of a render of the document.