These works again now after:

commit 8d837cbac41b121760207e784ba7b54f1ef84f32
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Thu Jan 17 19:02:48 2013 +0100

    assrender: reenable unit tests and up rank to primary again

commit 143c0cbd45008c208fe8f2201429adb54c101ad2
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Fri Jan 18 16:14:03 2013 +0100

    assrender: use overlay composition to blit subtitles on video frames
    
    Re-uses existing blitting code and also means we can support
    a lot more video formats out of the box, hence avoid unnecessary
    pixel format or colour space conversions.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=692012

commit 5f913665537e40758540eb8fa9858adace3650bd
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Thu Jan 17 18:48:12 2013 +0100

    assrender: render overlay composition if supported downstream
    
    This allows rendering ASS subtitles on top of video when
    using hardware-accelerated video decoders based on e.g.
    VA-API or VDPAU.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=678389
    https://bugzilla.gnome.org/show_bug.cgi?id=692012

commit 30222d608044bcd29537218309882f43f3c35963
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Mon Jan 21 10:35:49 2013 +0100

    assrender: add wait-text property
    
    This is the same property as in the pango element, to allow waiting for
    text on the text pad before rendering the video.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=692012

commit c78714ced3c46f4c0aa66dfdcc909baee2783fa4
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Fri Jan 18 15:25:48 2013 +0100

    assrender: fix some subtitles being ignored
    
    https://bugzilla.gnome.org/show_bug.cgi?id=692012

commit c16d41d7091f808d5c3b06b79160dbe187594ead
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Thu Jan 17 18:33:11 2013 +0100

    assrender: refactor code to avoid duplication
    
    https://bugzilla.gnome.org/show_bug.cgi?id=692012

commit 61976b7aa71ab55f0028a1d2fea888a072e945a0
Author: Arnaud Vrac <avrac@freebox.fr>
Date:   Thu Jan 17 18:26:08 2013 +0100

    assrender: proxy allocations on video pad
    
    Forward allocation queries on video sink pad instead
    of discarding them.
    
    This allows the upstream elements to use the allocation
    parameters or pools from the downstream elements like
    videosink.