Bug 961: Stabilize glXGetFBConfigAttrib(..) and glXChooseFBConfig(..) Usage Against OpenGL Bugs ; Fix glXGetFBConfigs

- glXChooseFBConfig(..)
  - Remove NULL FBConfig pointer from result in native code,
    which has been observed in Mesa 8.0.5-4 libgl1-mesa-swx11 (Debian-7).

- glXGetFBConfigs
  - Add manual implementation similar to glXChooseFBConfig

- glXGetFBConfigAttrib(..)
  - glXGetFBConfig(..)
     - Returns boolean reflecting success, don't throw exception - caller handles error
     - Caller ignore failure if not essential (i.e. already chosen config)