For a given DP, returns a list of renderer ids giving
    the renderers that support the source file type

def getRenderers(filename):
    """For a given DP, returns a list of renderer ids giving
    the renderers that support the source file type"""
    global available_renderers
    renderers = []
    for rdrid, (renderer, module) in available_renderers.items():
        try:
            priority = renderer.canRender(filename)
        except:
            print("""Error in renderer: %s.canRender("%s"):""" % (rdrid, filename))
            traceback.print_exc()
            priority = None
        if priority:
            renderers.append((priority, rdrid))
    # sort by priority
    renderers.sort(lambda a, b: cmp(a[0], b[0]))
    # return list of IDs. Note that "none" should always be available and working
    return [a[1] for a in renderers] or ["link"]