OCL: small fixes for Laplacian, Canny, etc

Also fixed extrapolation in sepFilter2D and eigenValsVecs.

check_regression=OCL*Laplacian*:OCL*Canny*:OCL*Corner*:OCL*Sobel*:OCL*GaussianBlur*:OCL*Scharr*
test_filter=OCL*Laplacian*:OCL*Canny*:OCL*Corner*:OCL*Sobel*:OCL*GaussianBlur*:OCL*Scharr*
build_examples=OFF
test_modules=imgproc
@akarsakov Oh. My mistake, i didn't realize local group size.
But may be we can check this on the host, and don't call OpenCL kernel for this small images?
I think we don't have any speedup for rework on GPU small images anyway, but additional call `clamp` for every point will reduce speed on the big images.
 May be change 
``` (_src.cols() >= kernelX.cols && _src.rows() >= kernelY.cols) ```
to something like
``` (_src.cols() >= max(tileSizeX, kernelX.cols) && _src.rows() >= max(tileSizeY, kernelY.cols))```
?