2.67 * fix thread-safety for GD::Simple %COLORS (#26 melak)

• fix arc start-angle docs, RT #123277 Andrew G Gray
  
• improve setBrush docs, RT #123194 Andrew G Gray
  
• improve StringFT docs, RT #123193
  
• replace MacOSX by darwin, and not by Mac OS X/macOS as suggested
  in PR #24
  
• add GD::Image->_file method as suggested in RT #60488 by Kevin Ryde,
  also the helper GD::supportsFileType