Blender Git Loki

Fix Broken Font Preview (reported over IRC by kopias, thanks).

Broken by font wrap commit rBf2341f829654c4dc97, there was actually two things here:
* Using non-initialized color (which lead to transparent drawing...), we need to use new
`blf_draw_buffer__start/end` helpers here too, made them shared internally.
* Using `draw_str_i18n_nbr` as `draw_str[i]` length, ugh! That's the number of utf8 glyphs of
translated string, not the length of untranslated string!

This fix must be backported to final 2.76.