Bit-exact version of RGB2Luv_b (opencv#9226)

* Imgproc_ColorLab_Full.accuracy test fixed

* Lab and Luv tests: rewritten, constants explained

* CV_ColorCvtBaseTest: added methods for 8u implementations

* Lab2RGB_b: bit-exactness enabled for all modes; non-vectorized code fixed to comply with vectorized

* srgb support added

* XYZ constants made softdouble

* bit-exact tests written for Lab

* ColorLab_full test fixed

* reverted: no 8u convertors for CV_ColorCvtBaseTest

* added checksum-based test for Lab bit-exactness

* extra declarations removed

* Lab test fix: stop at first mismatch

* test info output improved

* error message fixed

* lab_tetra squashed

* initial version is almost written

* unfinished work

* compilation fixed, to be debugged

* Lab test removed

* more fixes

* Luv2RGBinteger: channels order fixed

* Lab structs removed

* good trilinear interpolation added

* several fixes

* removed Luv2RGB interpolations, XYZ tables; 8-cell LUT added

* no_interpolate made 8-cell

* interpolations rewritten to 8-cell, minor fixes

* packed interpolation added for RGB2Luv

* tetra implemented

* removing unnecessary code

* LUT building merged

* changes ported to color.cpp

* minor fixes; try to suppress warnings

* fixed v range of Luv

* fixed incorrect src channel number

* minor fixes

* preliminary version of Luv2RGBinteger is done

* Luv2RGB_b is in progress

* XYZ color constants converted to softfloat

* Luv test: precision fixed

* Luv bit-exactness test added

* warnings fixed

* compilation fixed, error message fixed

* test_lab.cpp removed