Fix contrast and accumulation averaging

[Hpn4]

The new behavior is:

• On pipe refresh the accumulation queue is cleared.
• The autocontrast is applied (if enabled) if there is no accumulation, at the first image of the accumulation queue (to calibrate the contrast) and when the queue is full (to fine tune the contrast).
• The accumulation averaging is not done on all images but only on the images in the queue (no more black screen).