racket/gui Cocoa: repairs for Cairo and Pango

 * Fix a clipped-rendering problem for text in Cairo.

 * Fix Pango's CoreText back-end to support non-BMP characters.

   Note that Emoji characters still do not render. Cairo uses
   CGContextShowGlyphsWithAdvances() to draw glyphs, but it
   would need to use CTFontDrawGlyphs() to make Emoji work.
   (Mozilla has a patch to do that for some older version
   of Cairo, so look there if it seems worth doing one day.)

 * Disable Pango's CoreText font fallbacks in favor of the
   Racket-implemented fallback.

   This is not obviously a good idea, but it restore the
   `racket/draw` hack of prefering "Arial Unicode MS". Otherwise,
   various symbol glyphs are chosen badly, such as #\u273A,
   at least on my machine.

 * Drop the clusters argument to `pango_cairo_show_glyph_string`,
   which turns out to be unnecessary.