The documentation for `EXT_texture_from_pixmap` states that a
compositor should bind and release the texture each time:

    [...]
    glXBindTexImageEXT()

    <Do rendering/compositing>

    glXReleaseTexImageEXT()
    [...]

Fix the compositor to adhere to the specification.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>