G-API: Mosaic handle corner cases

* Handle corner cases

* Fix mosaic algo

* Fix bug with empty rects

* G-API: Added G-API Overview slides & its source code

- Sample code snippets are moved to separate files;
- Introduced a separate benchmark to measure Fluid/OpenCV
  performance;
- Added notes on API changes (it is still a 4.0, not a 4.2 talk!)
- Added a "Metropolis" beamer download-n-build script.

* G-API: Addressed review issues on G-API overview slides

* imgproc: Prevent 1B overrun of 8C3 SIMD optimization

The fourth value read via v_load_q is essentially ignored,
but can cause trouble if it happens to cross page boundaries.

The final few iterations may attempt to read the most extreme
elements of S, which will read 1B beyond the array in most
aligment cases. Dynamically compute the stop. This could be
hoised from the loop, but will require a more extensive change.

Likewise, cleanup the iteration increment statements to make
it more obvious they do channel count (3) elements per pass.

This should resolve #16137

* imgproc(resize): extra check

dnn(eltwise): fix handling of different number of channels

* dnn(test): reproducer for Eltwise layer issue from PR16063

* dnn(eltwise): rework support for inputs with different channels

* dnn(eltwise): get rid of finalize(), variableChannels

* dnn(eltwise): update input sorting by number of channels

- do not swap inputs if number of channels are same after truncation

* dnn(test): skip "shortcut" with batch size 2 on MYRIAD targets

G-API: Add default initialization for primitives

* Add ctors for primitives

* Add description for constructors

-c is required to avoid linking (and the associated missing "main" message)
when linker flags like "-Wl,-z,relro" are passed to GCC

G-API: Fix various issues for 4.2 release

* G-API: Fix issues reported by Coverity

- Fixed: passing values by value instead of passing by reference

* G-API: Fix redundant std::move()'s in return statements

Fixes #15903

* G-API: Added a smarter handling of Stop messages in the pipeline

- This should fix the "expected 100, got 99 frames" problem
- Fixes #15882

* G-API: Pass enum instead of GKernelPackage in Streaming test parameters

- Likely fixes #15836

* G-API: Address review issues in new bugfix comments

* G-API-NG/Docs: Added a tutorial page on interactive face detection sample

- Introduced a "--ser" option to run the pipeline serially for
  benchmarking purposes
- Reorganized sample code to better fit the documentation;
- Fixed a couple of issues (mainly typos) in the public headers

* G-API-NG/Docs: Reflected meta-less compilation in new G-API tutorial

* G-API-NG/Docs: Addressed review comments on Face Analytics Pipeline example

add CropAndResize layer for CUDA backend

* add CropAndResize layer

* process multiple channels per iteration

* Add eps error checking for approxPolyDP to allow sensible values only
for epsilon value of Douglas-Peucker algorithm.

* Review changes for PR