samples: add videocapture_camera sample

- show capturing information: width / height / fps
- show average FPS for cap.read()+imshow() via cv::getTickCount()
- optional frame processing code path

samples/cpp/videocapture_camera.cpp

+#include <opencv2/core.hpp>
+#include <opencv2/videoio.hpp>
+#include <opencv2/highgui.hpp>
+#include <opencv2/imgproc.hpp>  // cv::Canny()
+#include <iostream>
+
+using namespace cv;
+using std::cout; using std::cerr; using std::endl;
+
+int main(int, char**)
+{
+    Mat frame;
+    cout << "Opening camera..." << endl;
+    VideoCapture capture(-1); // open the first available camera
+    if (!capture.isOpened())
+    {
+        cerr << "ERROR: Can't initialize camera capture" << endl;
+        return 1;
+    }
+
+    cout << "Frame width: " << capture.get(CAP_PROP_FRAME_WIDTH) << endl;
+    cout << "     height: " << capture.get(CAP_PROP_FRAME_HEIGHT) << endl;
+    cout << "Capturing FPS: " << capture.get(CAP_PROP_FPS) << endl;
+
+    cout << endl << "Press 'ESC' to quit, 'space' to toggle frame processing" << endl;
+    cout << endl << "Start grabbing..." << endl;
+
+    size_t nFrames = 0;
+    bool enableProcessing = false;
+    int64 t0 = cv::getTickCount();
+    int64 processingTime = 0;
+    for (;;)
+    {
+        capture >> frame; // read the next frame from camera
+        if (frame.empty())
+        {
+            cerr << "ERROR: Can't grab camera frame." << endl;
+            break;
+        }
+        nFrames++;
+        if (nFrames % 10 == 0)
+        {
+            const int N = 10;
+            int64 t1 = cv::getTickCount();
+            cout << "Frames captured: " << cv::format("%5lld", (long long int)nFrames)
+                 << "    Average FPS: " << cv::format("%9.1f", (double)getTickFrequency() * N / (t1 - t0))
+                 << "    Average time per frame: " << cv::format("%9.2f ms", (double)(t1 - t0) * 1000.0f / (N * getTickFrequency()))
+                 << "    Average processing time: " << cv::format("%9.2f ms", (double)(processingTime) * 1000.0f / (N * getTickFrequency()))
+                 << std::endl;
+            t0 = t1;
+            processingTime = 0;
+        }
+        if (!enableProcessing)
+        {
+            imshow("Frame", frame);
+        }
+        else
+        {
+            int64 tp0 = cv::getTickCount();
+            Mat processed;
+            cv::Canny(frame, processed, 400, 1000, 5);
+            processingTime += cv::getTickCount() - tp0;
+            imshow("Frame", processed);
+        }
+        int key = waitKey(1);
+        if (key == 27/*ESC*/)
+            break;
+        if (key == 32/*SPACE*/)
+        {
+            enableProcessing = !enableProcessing;
+            cout << "Enable frame processing ('space' key): " << enableProcessing << endl;
+        }
+    }
+    std::cout << "Number of captured frames: " << nFrames << endl;
+    return nFrames > 0 ? 0 : 1;
+}