Support MSVC 2013 (#12486)

* Added CV_CONSTEXPR macro

* Utilize CV_NOEXCEPT and CV_CONSTEXPR

* Provides some Ptr<> logical operators

* added basic support for CV_16F (the new datatype etc.). CV_USRTYPE1 is now equal to CV_16F, which may break some [rarely used] functionality. We'll see

* fixed just introduced bug in norm; reverted errorneous changes in Torch importer (need to find a better solution)

* addressed some issues found during the PR review

* restored the patch to fix some perf test failures

* may be an typo fix

* remove identical branch,may be paste error

* add parentheses around macro parameter

* simplify if condition

* check malloc fail

* change the condition of branch removed by commit 3041502861318c85d901e8c08226ff67898c77ed

* rewrote Mat::convertTo() and convertScaleAbs() to wide universal intrinsics; added always-available and SIMD-optimized FP16<=>FP32 conversion

* fixed compile warnings

* fix some more compile errors

* slightly relaxed accuracy threshold for int->float conversion (since we now do it using single-precision arithmetics, not double-precision)

* fixed compile errors on iOS, Android and in the baseline C++ version (intrin_cpp.hpp)

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix the custom AVX2 builder test failures (false alarms)

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* seemingly disabled false alarm warning in surf.cpp; increased tolerance thresholds in the tests for SolvePnP and in DNN/ENet

POD structures can't have constructors.

Commits:
- 09837928
- 10fb88d0

Excluded changes with std::atomic (C++98 requirement)

- support passing of `std::vector<KeyPoint>` via InputArray

* fix some static analyzer warnings

* fix some static analyzer warnings

* fix race condition of workthread control

* Bit-exact GaussianBlur reworked to use wide intrinsics

* Added v_mul_hi universal intrinsic

* Removed custom SSE2 branch from bit-exact GaussianBlur

* Removed loop unrolling for gaussianBlur horizontal smoothing

except header file with conversion routines (will be removed in OpenCV 4.0)

* Update core.hpp

* Update imgproc.hpp

* Update ImgprocTest.java

* Update CameraCalibrator.java

* Update OnCameraFrameRender.java

* Update FindContoursDemo.java

* Update IntroductionToSVMDemo.java

* Update NonLinearSVMsDemo.java

* Update IntroductionToPCADemo.java

* Update Smoothing.java

* Update MainActivity.java

* Update CalcBackProjectDemo1.java

* Update CornerSubPixDemo.java

* Update CornerDetectorDemo.java

* Update GoodFeaturesToTrackDemo.java

Intrinsics must be effective, so don't declare FP16 type/operations if there is no native support.

- CV_FP16: supports load/store into/from float32
- CV_SIMD_FP16: declares FP16 types and native FP16 operations

Multi-argument CV_Assert() is accessible via CV_Assert_N() (with malformed messages).

OpenCV 3.4.3

remove from DNN module headers

All vx_call() must be wrapped into own simd128/simd256/simd512 namespace

```
namespace CV__SIMD_NAMESPACE {
... vx_call declaration is here ...
}
```

Fix for SSE2 intrinsics problem in the part of saturation arithmetic processing during 32s->16u packed conversion -
for some big negative values less than -INT_MAX+32767 the sign of the numbers is lost due to overflow that leads to
incorrect saturation to MAX value, instead of zero.
The issue is not reproduced with CV_ENABLED_INTRINSICS=OFF

- tests for both SIMD128 / SIMD256
- different dispatched + baseline(SIMD128) intrinsics

Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`