Removed redundant clFinish() after clEnqueueUnmapMemObject()

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adding fourcc to VideoWriter docs

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Merge pull request #3167 from akarsakov:ocl_rm_clFinish
Merge pull request #3182 from berak:videowriter_docfix_30
Fixed bad non-ASCII character in how_to_scan_images.cpp which causes a compile error
Remove mul24 since id can be larger 2^23
Merge pull request #3195 from akarsakov:ocl_rm_mul24
Merge pull request #3192 from jkua:master