'Render a single tile, return a SaveableResponse instance.'

def renderTile(self, width, height, srs, coord):

    nw = self.layer.projection.coordinateLocation(coord)
    se = self.layer.projection.coordinateLocation(coord.right().down())
    ul = self.mercator.locationProj(nw)
    lr = self.mercator.locationProj(se)
    bbox = ('ST_SetSRID(ST_MakeBox2D(ST_MakePoint(%.6f,    %.6f),    ST_MakePoint(%.6f,    %.6f)),    900913)' % (ul.x, ul.y, lr.x, lr.y))
    clip = ((self.clipping and Polygon([(ul.x, ul.y), (lr.x, ul.y), (lr.x, lr.y), (ul.x, lr.y)])) or None)
    db = _connect(self.dbdsn).cursor(cursor_factory=RealDictCursor)
    db.execute(self.query.replace('!bbox!', bbox))
    rows = db.fetchall()
    db.close()
    response = {'type': 'FeatureCollection', 'features': []}
    for row in rows:
        feature = row2feature(row, self.id_field, self.geometry_field)
        try:
            geom = shape2geometry(feature['geometry'], self.mercator, clip)
        except _InvisibleBike:
            pass
        else:
            feature['geometry'] = geom
            response['features'].append(feature)
    return SaveableResponse(response, self.indent, self.precision)
