made improvements in ECC code (comments from https://github.com/Itseez/opencv/pull/3845), as well as minor tweak in pthread's based parallel for.

Tests of the mask are also included.

This is useful for registering a non-square image against a non-square
template.

This also needs to relax a sanity check as per
https://github.com/Itseez/opencv/pull/3851

The matrix templateZM needs to be initialized because otherwise
uninitialized values leak into the correlation in:

    const double correlation = templateZM.dot(imageWarped)

In the worst case this will lead the correlation to be NaN ruining the
whole routine. The subtraction does not initialize templateZM due to the
mask.

Unfortunately, the uninitialized values (by altering the correlation)
have the side effect of dragging out the computation a little longer
giving a slightly better error bound. This means that fixing this bug
breaks perf_ecc where

    SANITY_CHECK(warpMat, 1e-3);

is just a little too tight and happens to work due to the uninitialized
values. Since this is a performance not a accuracy test I think it is OK
to just relax the error bound a little bit (the tight error bound being
after all the result of a bug).

quickly corrected the previous refactoring of features2d: moved from set(SOME_PROP, val) to setSomeProp(val)

IPP can be switched on and off on runtime;

Optional implementation collector was added (switched off by default in CMake). Gathers data of implementation used in functions and report this info through performance TS;

TS modifications for implementations control;

1. someMatrix.data -> someMatrix.prt()
2. someMatrix.data + someMatrix.step * lineIndex -> someMatrix.ptr( lineIndex )
3. (SomeType*) someMatrix.data -> someMatrix.ptr<SomeType>()
4. someMatrix.data -> !someMatrix.empty() ( or !someMatrix.data -> someMatrix.empty() ) in logical expressions

ECC patch by the author (G. Evangelidis); fixed some OCL Farneback optical flow test failures on Mac

replaces tabs with spaces

fixed slow down in getRectSubPix (original patch by Markus JR Mueller); removed some unused variables and functions

changed the way the mean value for each pixel is assigned in the output image.

modified BackgroundSubtractorMOG2::getBackgroundImage so that it can now work with gray-level images.

1. decrease branch number in CL code by replacing them into weights
2. decrease local mem pressure in reduce operation by using private variables
3. decrease image sampler pressure by caching data into local memory
4. remove unnecessary sync point on the HOST side.

Fixed an issue with weight assignment causing the resulting GMM weights to be unsorted in BackgroundSubtractorMOG2