Code review comment for lp:~kdub/mir/fix-1413211

The compositor jerkiness when touchspots are on (lp: #1373692) and this bug (lp: #1413211) are two different root causes. This one is a lockup because another part of the system tries to write to a buffer that is onscreen, and the android code disallows this to avoid tearing on the cursor image. The other bug's root cause is more that certain android devices don't have any fences or waits that stop the composition threads from spiking if we're trying to post the exact same thing twice in a row.