libcamera: request: Add Request::cancel()

Add a cancel() function to the Request class that allows to forcefully
complete the request and its associated buffers in error state.

Only pending requests can be forcefully cancelled. Enforce that
by asserting the request state to be RequestPending.

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

include/libcamera/request.h

@@ -65,6 +65,7 @@ private:
 	friend class PipelineHandler;
 
 	void complete();
+	void cancel();
 
 	bool completeBuffer(FrameBuffer *buffer);