glx-tfp.c is almost certainly broken

It does "bind tex image, draw, release tex image", and then expects the contents of the texture to be what it drew. That's exactly not what the spec says happens:

    The contents of the texture after the drawable has been bound are defined
    as the result of all rendering that has completed before the call to
    glXBindTexImageEXT.  In other words, the results of any operation which
    has caused damage on the drawable prior to the glXBindTexImageEXT call
    will be represented in the texture.