Feedback is a GL render mode.
The mode is selected by calling
glRenderMode with GL_FEEDBACK.
When the GL is in feedback mode,
no pixels are produced by rasterization.
information about primitives that would have been rasterized
is fed back to the application using the GL.
See the glFeedbackBuffer reference page for a description of the
feedback buffer and the values in it.
glPassThrough inserts a user-defined marker in the feedback buffer
when it is executed in feedback mode.
token is returned as if it were a primitive;
it is indicated with its own unique identifying value:
The order of glPassThrough commands with respect to the specification
of graphics primitives is maintained.