libcamera: buffer: Add const accessor to Buffer planes

In order to inspect planes of a const Buffer, add a const accessor.

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

src/libcamera/buffer.cpp

@@ -181,6 +181,12 @@ void *Plane::mem()
  * image format is multi-planar.
  */
 
+/**
+ * \fn BufferMemory::planes() const
+ * \brief Retrieve the planes within the buffer
+ * \return A const reference to a vector holding all Planes within the buffer
+ */
+
 /**
  * \fn BufferMemory::planes()
  * \brief Retrieve the planes within the buffer