[sensor] Add a field nb_points to DetphCamera

It is now useful, as the the buffer does not contain anymore the full
image, but only the points which are in the 'normal range' of the camera
(stricly inferior to far clip so).
[middleware/pocolibs] Update stereopixel output to take in account 76380da0
[middleware/pocolibs] Improve stereopixel overlay

It now matches properly the expected output of stereopixel::Compute