libcamera: controls: Add sensor black levels reporting through metadata

The black levels will be used when saving RAW data in DNG containers.

Four values are reported, one for each channel, in the order R, Gr, Gb
and B. The values are reported as being out of a 16-bit pixel range
(a fully saturated pixel would be 65535), so may need subsequent
re-scaling, depending on use.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/control_ids.yaml

@@ -230,4 +230,13 @@ controls:
         Specify a fixed saturation parameter. Normal saturation is given by
         the value 1.0; larger values produce more saturated colours; 0.0
         produces a greyscale image.
+
+  - SensorBlackLevels:
+      type: int32_t
+      description: |
+        Reports the sensor black levels used for processing a frame, in the
+        order R, Gr, Gb, B. These values are returned as numbers out of a 16-bit
+        pixel range (as if pixels ranged from 0 to 65535). The SensorBlackLevels
+        control can only be returned in metadata.
+      size: [4]
 ...