Code review comment for lp://qastaging/~alan-griffiths/mir/fix-1535894-alt

OK, assuming Android has some magic workaround built-in to cope with posting a display group where only half of that group has been rendered, what we actually want instead of this branch is:

bool composited = false
for each compositor:
   composited |= compositor->composite();
if (composited)
   group.post();

That also means you can keep the frame counting logic where it is in the scene. No need to duplicate it into each compositor implementation.

But if Android can't handle half the monitors being rendered and then being told to post the whole group, then we actually can't safely do any of this at all...