scaling a surface

local function draw(self, content, cr, width, height)
    local c = self._private.client[1]
    local s = surface(c.content)

    local geo = c:geometry()

    local scale = math.min(width/geo.width, height/geo.height)

    local w, h = geo.width*scale, geo.height*scale

    local dx, dy = (width-w)/2, (height-h)/2

    cr:translate(dx, dy)

    shape.rounded_rect(cr, w, h)
    cr:clip()

    cr:scale(scale, scale)

    cr:set_source_surface(s)
    cr:paint()
end