Since commit e30eb788 `ClutterFrameClock` assumes that a valid CPU time
implies timestamp query support, which is also checked in
`cogl_onscreen_egl_swap_buffers_with_damage()`.

Unconditionally setting the CPU time on direct scanout meant that the
compositing path would be stuck on the last (direct scanout optimized)
result on GL implementations without timestamp query support since.

be0aa297 (clutter/frame-clock: Avoid rapidly toggling dynamic max render time)

Fix that by explicitly marking the gpu rendering duration as valid when
querying the GPU timestamps is supported and check for it ClutterFrameClock.

Fixes: 56580ea7 ("backends/native: Assume zero rendering time for direct scanout buffers")
Part-of: <!3655>