Added implementations for eglQueryDisplayAttribEXT, eglQueryDeviceAttribEXT,
and eglQueryDeviceStringEXT.

Defined a new EGL_DEVICE_INDEX attribute for eglGetPlatformDisplay that uses
a device based on its index. This will allow us to test the case where
eglQueryDisplayAttribEXT returns an EGLDeviceEXT handle that libglvnd hasn't
seen before.