Fix most of 'disappearing' first letter in right-aligned labels.

BLF' blf_font_width_to_strlen() could easily generate strings with up to
nearly two pixels length over requested limit!

Note that the fiddling between floats and ints values make things really
confusing here... :/

There is still a few limit cases where, even though computed str length
is now always below reauested limit, we still get first letter
disappearing, no idea why currently.

source/blender/editors/interface/interface_widgets.c

@@ -1500,6 +1500,8 @@ float UI_text_clip_middle_ex(
 		BLF_disable(fstyle->uifont_id, BLF_KERNING_DEFAULT);
 	}
 
+	BLI_assert(strwidth <= okwidth);
+
 	return strwidth;
 }