Blender Git Loki

BLI_string_utf8: simplify utf8 stepping logic

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.