[Vexi-core] Font Effects / underline

[Charlie <cha...@gm...>]

Been looking into getting underlined text; it seems that position/width
hints are returnable from the font file via freetype but it must be drawn
manually.

Source:
http://www.freetype.org/freetype1/docs/faq/freetype1.txt

    Underlining  and stroking, are  not really  part of  the glyphs.
    They're simply lines that are  printed on the glyph after it has
    been rendered.  Each TrueType  file provides, in its OS/2 table,
    which  is  accessible  through  the face  object  properties  in
    FreeType, several  values that define the position  and width of
    those lines, in notional font units.

    If you  want to use them,  you'll have to scale  these values to
    your current instance/point size, then draw the lines yourself.

These values seem to be available from the font's "global design metrics"
and are called ' underline_position ' (relative to the baseline/bottom -
negative is below) and ' underline_thickness '. It seems we need to modify
the core to expose these values.

Source:
http://www.freetype.org/freetype2/docs/tutorial/step2.html

 - C