Blender Git Loki

BLI_string_utf8: add buffer size arg to BLI_str_utf8_from_unicode

Besides helping to avoid buffer overflow errors this reduces complexity
of BLI_str_utf32_as_utf8 which needed a special loop for the last 6
characters to avoid writing past the buffer bounds.

Also add BLI_str_utf8_from_unicode_len which only returns the length.