`to_strlen` just benefits from using pre-computed kerning table for
ascii chars (gives about 30% speed improvements).
`to_rstrlen` was re-written and heavily simplified, basically using same
logic as `to_strlen`, and `BLI_str_find_prev_char_utf8()` to loop
backward in the string, instead of looping forwards the whole string,
storing each gliph's width in temp array, and looping backward on that
temp array to find final string matching expected width. Gives about 70%
speed improvements!
And both functions can now share their core logic.
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