Code review comment for lp:~alan-griffiths/mir/partial-fix-for-1532202

Clearing the contents appropriately will correct artifacts present in the buffer from previous renders, so we need some way to know whether a clear is needed or not.
So maybe something like:

if (renderlist.empty() && clear) return;
(where clear is a variable that gets set after the first clear, and unset when the renderables are drawn)
would work.

Still thinking what could be the root cause. If the swapping (ie, producing gpu traffic) is slowing us down, maybe its a bus traffic jam issue. (and if thats the case, maybe the n7 wouldn't have the problem)