Added credit.
Fixed constness of function parameters.
Fixed refresh of debug context attribute.
Fixed documentation copypaste error.
Made GLX 1.3 the minimum supported version.
Made use of Win32 W-suffixed symbols explicit.
Further separation of window and context.

The context related parts of _GLFWwndconfig have been moved to
_GLFWctxconfig and given better names.  Window hint and attribute
members have been renamed to match.
Added missing link flags for threaded example.
Moved some VC++ specific hacks to platform header.
Added missing suffix for WNDCLASSW.
Fixes for legacy MinGW 4.2.
Removed stale comment.
Fixed link flags for threaded example.
Updated broken RandR gamma comment.