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>

include/libcamera/buffer.h

@@ -40,6 +40,7 @@ private:
 class BufferMemory final
 {
 public:
+	const std::vector<Plane> &planes() const { return planes_; }
 	std::vector<Plane> &planes() { return planes_; }
 
 private: