v0.21.0

• Added treat_as_current function.
• Breaking: Replaced CreationErrorPair enum variant with CreationErrors.
• Added Clone to ContextBuilder.
• Added headless example.
• Removed internal code relating to libcaca.
• Implemented Debug on all public facing types.
• Dropping contexts on platforms using egl and/or glx no longer resets the
  current context, if the context dropped wasn't the current context.
• Added context sharing support to MacOS.
• Breaking: Removed ContextTrait.
• Breaking: Renamed OsMesaContextExt to HeadlessContextExt. Added functions
  for using egl-surfaceless.
• Breaking: Changed WindowedContext and RawContext into typedefs of
  ContextWrapper.
• Breaking: Removed new_windowed and new_headless from WindowedContext
  and Context, respectively.
• Breaking: Added two new types, NotCurrent and PossiblyCurrent,
  which RawContext, WindowedContext, ContextBuilder and Context are now
  generic over.
• Added {make,treat_as}_not_current function to {Raw,Windowed,}Context.
• We now load libGL.so instead of libGLX.so.
• Fixed bug where we drop the hidden window belonging to a headless context on
  on X11 and/or Wayland before the actual context.
• "Fixed" bug where we will close EGLDisplays while they are still in use by
  others. Angry and/or salty rant can be found in glutin/src/api/egl/mod.rs,
  you can't miss it.
• Breaking: WindowedContexts now deref to Context, not Window.
  Please use .window() to access the window.