Code review comment for lp:~kdub/mir/fix-1378326

So, in this mixed mode composition, we have a portion of the screen we have to render via gl, and a portion that we have to clear. In our use cases, we still push out all the pixels on the display, but just save in terms of the number of pixels that have been rendered vs the number that have been fast-cleared. So yes, there's still room for optimization, but it seems more of a subset of trying to do partial screen updates (which we don't try to do at the moment)