ipa: ipu3: Derive ipu3::algorithms::Agc from AgcMeanLuminance

In preparation for switching to a derivation of AgcMeanLuminance, add
a function to parse and store the statistics for easy retrieval in an
overriding estimateLuminance() function.

Now that we have a MeanLuminanceAgc class that centralises our AEGC
algorithm, derive the IPU3's Agc class from it and plumb in the
necessary framework to enable it to be used. For simplicity's sake
this commit switches the algorithm to use the derived class, but
does not remove the bespoke functions at this time.

Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/ipa/ipu3/ipa_context.cpp

@@ -47,6 +47,9 @@ namespace libcamera::ipa::ipu3 {
  *
  * \var IPAContext::activeState
  * \brief The current state of IPA algorithms
+ *
+ * \var IPAContext::ctrlMap
+ * \brief A ControlInfoMap::Map of controls populated by the algorithms
  */
 
 /**