ocl: avoid unnecessary loading/initializing OpenCL subsystem

If there are no OpenCL/UMat methods calls from application.

OpenCL subsystem is initialized:
- haveOpenCL() is called from application
- useOpenCL() is called from application
- access to OpenCL allocator: UMat is created (empty UMat is ignored) or UMat <-> Mat conversions are called

Don't call OpenCL functions if OPENCV_OPENCL_RUNTIME=disabled
(independent from OpenCL linkage type)

modules/video/src/lkpyramid.cpp

@@ -1226,7 +1226,7 @@ void SparsePyrLKOpticalFlowImpl::calc( InputArray _prevImg, InputArray _nextImg,
 {
     CV_INSTRUMENT_REGION()
 
-    CV_OCL_RUN(ocl::useOpenCL() &&
+    CV_OCL_RUN(ocl::isOpenCLActivated() &&
                (_prevImg.isUMat() || _nextImg.isUMat()) &&
                ocl::Image2D::isFormatSupported(CV_32F, 1, false),
                ocl_calcOpticalFlowPyrLK(_prevImg, _nextImg, _prevPts, _nextPts, _status, _err))