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/stitching/src/blenders.cpp

@@ -478,7 +478,7 @@ void MultiBandBlender::feed(InputArray _img, InputArray mask, Point tl)
     {
         Rect rc(x_tl, y_tl, x_br - x_tl, y_br - y_tl);
 #ifdef HAVE_OPENCL
-        if ( !cv::ocl::useOpenCL() ||
+        if ( !cv::ocl::isOpenCLActivated() ||
              !ocl_MultiBandBlender_feed(src_pyr_laplace[i], weight_pyr_gauss[i],
                     dst_pyr_laplace_[i](rc), dst_band_weights_[i](rc)) )
 #endif
@@ -633,7 +633,7 @@ void normalizeUsingWeightMap(InputArray _weight, InputOutputArray _src)
 #endif
 
 #ifdef HAVE_OPENCL
-    if ( !cv::ocl::useOpenCL() ||
+    if ( !cv::ocl::isOpenCLActivated() ||
             !ocl_normalizeUsingWeightMap(_weight, _src) )
 #endif
     {