Check for other devices in vtkEGLRenderWindow

This change implements a new environment variable: VTK_EGL_DEVICE_INDEX. This forces the VTK to initialize the device at the specified index. If this is not set, then the system will try to initialize VTK_DEFAULT_EGL_DEVICE_INDEX first and then all other devices found next. If no device can be initialized, then VTK exits.

This is part of getting GLVND EGL to work with VTK.