uiUnderlineSuggestion with uiUnderlineColorCustom

`uiUnderlineSuggestion` doesn't work with `uiUnderlineColorCustom`:

In the drawtext example:
```c
uiAttributedStringAppendUnattributed(attrstr, "Furthermore, there are attributes allowing for ");
attr = uiNewUnderlineAttribute(uiUnderlineSuggestion);
attr2 = uiNewUnderlineColorAttribute(uiUnderlineColorSpelling, 0, 0, 0, 0);
appendWithAttribute("special underlines for indicating spelling errors", attr, attr2);
uiAttributedStringAppendUnattributed(attrstr, " (");
attr = uiNewUnderlineAttribute(uiUnderlineSuggestion);
attr2 = uiNewUnderlineColorAttribute(uiUnderlineColorCustom, 0.1, 0.1, 0.9, 1);
appendWithAttribute("and other types of errors", attr, attr2);
uiAttributedStringAppendUnattributed(attrstr, ") ");
```