OpenCv.js docker container build instructions (#15221)

* emscripten docker container

* docker

* typo

* typo

* whitespace issue

Fix build for UWP (#15207)

* Guard non-WinRT calls to fix UWP build

* Remove unnecessary guard for WinRT

G-API: verify y offset and height of Fluid output ROIs (#15190)

* G-API: validate GFluidOutputRoi y and height

* Rearrange tested ROIs

* G-API: fix fluid reshape with unused nodes

* Update test names

* Add FIXME for future improvement

* Add high level API

* Fix Model

* Add DetectionModel

* Add ClassificationModel

* Fix classify

* Add python test

* Fix pytest

* Fix comments to review

* Fix detect

* Fix docs

* Modify DetectionOutput postprocessing

* Fix test

* Extract ref boxes

* Fix draw rect

* fix test

* Add rect wrap

* Fix wrap

* Fix detect

* Fix Rect wrap

* Fix OCL_FP16

* Fix MyriadX

* Fix nms

* Fix NMS

* Fix coords

This fixes a compiler error on MSVC with Unreal Engine (at least).

OpenCV 4.1.1

OpenCV 3.4.7

* Unroll multiply and add instructions in dotProd_32f - 35% faster.

* Eliminate unnecessary v_reduce_sum instructions.

* G-API: fix GOCLExecutable issue with UMat lifetime

Add tests on initialized/uninitialized outputs for all
backends

* Use proper clean-up procedure for magazine

* Rename InitOut test and reduce tested sizes

* Enable output allocation test

G-API: GAPI_TRANSFORM internal functionality rework (#14952)

* Change internal pattern and substitute signatures and refactor tests

* Enhance GArrayU with type-checker function

Add a couple of new tests on GAPI_TRANSFORM

* Improve support for Windows Embedded Compact

* Remove redundant set(WINCE true) and format CMake