libcamera: request: Make Stream pointer const

The Stream pointer just acts as a key in the Request object. There is no
good use-case to modify a stream from a pointer retrieved from the
Request, make it const. This allows pipeline handlers to better express
that the Stream pointer is retrieved in a Request should just be treated
as a key.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

test/camera/buffer_import.cpp

@@ -51,7 +51,7 @@ protected:
 		completeRequestsCount_++;
 
 		/* Create a new request. */
-		Stream *stream = buffers.begin()->first;
+		const Stream *stream = buffers.begin()->first;
 		FrameBuffer *buffer = buffers.begin()->second;
 
 		request = camera_->createRequest();