Added support for YUV2RGB[A]_NV21 and YUV2BGR[A]_NV21 conversion
Added OCL code for YUV2BGR_YV12 and YUV2BGR_IYUV color conversions
Added OCL code for YUV2GRAY_420 color conversion
Added OCL code for RGB[A]|BGR[A] -> YUV_[YV12|IYUV] color conversion
Added OCL code for YUV422 -> RGB[A]|BGR[A] color conversion
Used direct float calculations
Optimize OpenCL version of conversScaleAbs function
Basic doxygen documentation support

- updated existing Doxyfile.in
- added corresponding cmake instructions
- added some specific files (layout, icon)
- clean existing doxygen warnings
Fixed imgcodecs tests

- Imgcodecs_Image, write_imageseq - assuming JPEG images have losses
- Imgcodecs_Drawing, c_regression - replaced assert calls
- Imgcodecs_Drawing - exact comparison with reference image
Disabled videoio_video_prop_fps test

This test does not return correct values with some software configurations.
Possible problem with ffmpeg on buildbot's linux-slave.
naive implementation
use local memory
use vectors
Some small fixes
Added loading 4 pixels in line instead of 2 to RGB[A] -> YUV(420) kernel
Added optimized loading to YUV2RGB_422 kernel
use LOCAL_SIZE+1
Don't process empty src image in GFTT
Merge pull request #3379 from akarsakov:gftt_empty_src
Merge pull request #3159 from ElenaGvozdeva:ocl_gemm