tracking: rework tracking API

- simplify Tracker API
- fix perf tests (don't measure video decoding)

modules/tracking/perf/perf_main.cpp

@@ -3,4 +3,19 @@
 // of this distribution and at http://opencv.org/license.html.
 #include "perf_precomp.hpp"
 
-CV_PERF_TEST_MAIN(tracking)
+static
+void initTrackingTests()
+{
+    const char* extraTestDataPath =
+#ifdef WINRT
+        NULL;
+#else
+        getenv("OPENCV_DNN_TEST_DATA_PATH");
+#endif
+    if (extraTestDataPath)
+        cvtest::addDataSearchPath(extraTestDataPath);
+
+    cvtest::addDataSearchSubDirectory("");  // override "cv" prefix below to access without "../dnn" hacks
+}
+
+CV_PERF_TEST_MAIN(tracking, initTrackingTests())