clutter/stage-cogl: Don't skip over the next frame

The `last_presentation_time` is usually a little in the past (although
sometimes in the future depending on the driver). When it's over 2ms
(`sync_delay`) in the past that would trigger the while loop to count up so
that the next `update_time` is in the future.

The problem with that is for common values of `last_presentation_time`
which are only a few milliseconds ago, incrementing `update_time` by
`refresh_interval` also means counting past the next physical frame that
we haven't rendered yet. And so mutter would skip that frame.

**Example**

Given:
```
  last_presentation_time = now - 3ms
  sync_delay = 2ms
  refresh_interval = 16ms
  next_presentation_time = last_presentation_time + refresh_interval
                         = now + 13ms

          -3ms now        +13ms           +29ms           +45ms
        ----|--+------------|---------------|---------------|----
            :               :
  last_presentation_time  next_presentation_time
```

Old algorithm:
```
  update_time = last_presentation_time + sync_delay
              = now - 1ms
  while (update_time < now)
        (now - 1ms   < now)
    update_time = now - 1ms + 16ms
  update_time = now + 15ms
  next_presentation_time = now + 13ms
  available_render_time = next_presentation_time - max(now, update_time)
                        = (now + 13ms) - (now + 15ms)
                        = -2ms  so the next frame will be skipped.

          -3ms now        +13ms           +29ms           +45ms
        ----|--+------------|-+-------------|---------------|----
            :               : :
            :               : update_time (too late)
            :               :
  last_presentation_time  next_presentation_time (a missed frame)

```

New algorithm:
```
  min_render_time_allowed = refresh_interval / 2
                          = 8ms
  max_render_time_allowed = refresh_interval - sync_delay
                          = 14ms
  target_presentation_time = last_presentation_time + refresh_interval
                           = now - 3ms + 16ms
                           = now + 13ms
  while (target_presentation_time - min_render_time_allowed < now)
        (now + 13ms - 8ms < now)
        (5ms < 0ms)
    # loop is never entered
  update_time = target_presentation_time - max_render_time_allowed
              = now + 13ms - 14ms
              = now - 1ms
  next_presentation_time = now + 13ms
  available_render_time = next_presentation_time - max(now, update_time)
                        = (now + 13ms) - now
                        = 13ms  which is plenty of render time.

          -3ms now        +13ms           +29ms           +45ms
        ----|-++------------|---------------|---------------|----
            : :             :
            : update_time   :
            :               :
  last_presentation_time  next_presentation_time
```

The reason nobody noticed these missed frames very often was because
mutter has some accidental workarounds built-in:

 * Prior to 3.32, the offending code was only reachable in Xorg sessions.
   It was never reached in Wayland sessions because it hadn't been
   implemented yet (till e9e4b2b7).

 * Even though Wayland support is now implemented the native backend
   provides a `last_presentation_time` much faster than Xorg sessions
   (being in the same process) and so is less likely to spuriously enter
   the while loop to miss a frame.

 * For Xorg sessions we are accidentally triple buffering (#334). This
   is a good way to avoid the missed frames, but is also an accident.

 * `sync_delay` is presently just high enough (2ms by coincidence is very
   close to common values of `now - last_presentation_time`) to push the
   `update_time` into the future in some cases, which avoids entering the
   while loop. This is why the same missed frames problem was also noticed
   when experimenting with `sync_delay = 0`.

v2: adjust variable names and code style.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=789186
       and most of GNOME/mutter#571

GNOME/mutter!520
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