There were multiple utf8 functions which treated
errors slightly differently.
Split BLI_str_utf8_as_unicode_step into two functions.
- BLI_str_utf8_as_unicode_step_or_error returns error value
when decoding fails and doesn't step.
- BLI_str_utf8_as_unicode_step always steps forward at least one
returning the byte value without decoding
(needed to display some latin1 file-paths).
Font drawing uses BLI_str_utf8_as_unicode_step and no longer
check for error values.
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