libcamera: v4l2_controls: Turn V4L2ControlInfoMap into a class

In preparation for extending V4L2ControlInfoMap with control idmap
support, turn it into a real class. Make it inherit from std::map<>
instead of wrapping it to keep the API simple.

V4L2ControlInfoMap is meant to be constructed with a set of control
info, and never modified afterwards. To ensure this, inherit from
std::map<> with private access specifier, and explicitly expose the
std::map<> methods that do not allow insertion or removal of elements. A
custom move assignment operator is implemented to allow efficient
construction of the V4L2ControlInfoMap.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/v4l2_controls.cpp

@@ -156,10 +156,28 @@ V4L2ControlInfo::V4L2ControlInfo(const struct v4l2_query_ext_ctrl &ctrl)
  */
 
 /**
- * \typedef V4L2ControlInfoMap
+ * \class V4L2ControlInfoMap
  * \brief A map of control ID to V4L2ControlInfo
  */
 
+/**
+ * \brief Move assignment operator from plain map
+ * \param[in] info The control info map
+ *
+ * Populate the map by replacing its contents with those of \a info using move
+ * semantics. Upon return the \a info map will be empty.
+ *
+ * This is the only supported way to populate a V4L2ControlInfoMap.
+ *
+ * \return The populated V4L2ControlInfoMap
+ */
+V4L2ControlInfoMap &V4L2ControlInfoMap::operator=(std::map<unsigned int, V4L2ControlInfo> &&info)
+{
+	std::map<unsigned int, V4L2ControlInfo>::operator=(std::move(info));
+
+	return *this;
+}
+
 /**
  * \class V4L2Control
  * \brief A V4L2 control value