libcamera: Add gamma control id

A camera gamma of roughly 2.2 is necessary to produce correct output
images on a standard monitor. Add a control for that.

Further information is available here:
https://en.wikipedia.org/wiki/SRGB
https://www.cambridgeincolour.com/tutorials/gamma-correction.htm

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

src/libcamera/control_ids_core.yaml

@@ -865,4 +865,11 @@ controls:
           description: |
             This is a long exposure image.
 
+  - Gamma:
+      type: float
+      description:  |
+        Specify a fixed gamma value. Default must be 2.2 which closely mimics 
+        sRGB gamma. Note that this is camera gamma, so it is applied as 
+        1.0/gamma
+
 ...