RGB to/from Gray rewritten to wide intrinsics (#13379)

* 5x5 to RGB added

* RGB25x5 added

* Gray2RGB added

* Gray2RGB5x5 added

* vx_set moved out of loops

* RGB5x52Gray added

* RGB2Gray written

* warnings fixed (int -> (u)short conversion)

* warning fixed

* warning fixed

* "i < n-vsize+1" to "i <= n-vsize"

* RGBA2mRGBA vectorized

* try to fix ARM builds

* fixed ARM build for RGB2RGB5x5

* mRGBA2RGBA: saturation, vectorization

* fixed CL implementation of mRGBA2RGBA (saturation added)