move client post-update adding into render function

move client post-update adding into render function

in some cases, it's possible for a client which expects to render on
the next frame to actually render on the frame after. in these cases,
the compositor must not clear the pixmap image until after the render
has occurred in order to avoid inaccuracies. for this reason, the best
place to flag a client for post-render work is at the time of the client's
render

ref T2762
ref D3120