Code review comment for lp:~dandrader/mir/switchingBundle_lp1270964

Since 0 is a valid value it may come up again when our global/local frameno counts wrap around. So we either:
1. Need to allow it (proposed solution)
2. Make the value invalid and ensure 100% that it never comes up during normal operation

I am OK with (1). It allows the user to start with any initial value they want and this is in IMO a better interface since less is expected from the user and there is no opportunity for error.

That being said, I would also be happy with something like the following for approach (2):

struct FrameNumber
{
    void operator++() { if (++number == invalid_value) ++number; }
    int operator int() { return number; }

    static int const invalid_value = 0;

private:
    int number = invalid_value + 1;
};

BufferBundle::compositor_acquire(FrameNumber frameno);