clamp coverage value in text.rs between 0.0 and 1.0;

[asibahi]

Very minimal change. I had copied the code from this crate to use in my own projects, and found out that sometimes ab_glyph gives a higher coverage value than 1.0 when there are overlapping glyphs (like in Arabic.) The same change worked in my code. You can find the relevant issue here: alexheretic/ab-glyph#101

[theotherphil]

Thanks!