Fix the character index used for getting a glyph attribute in basic shaping

[dekrain]

I am using glyphon, based on cosmic-text, to render text in my game, and using basic shaping for most buffers.
After updating, color variations in a buffer stopped working. I found out that a change in #202 was introduced that fetches the attribute data per glyph instead of per run, but it uses a wrong index (index of the code point inside the run instead of buffer text's position). I am unsure how that worked out in their scenario, my guess is that they only used spanned attributes at the beginning of buffers, but I fixed it now.