If we have perfect matches (min_dist == 0.0), then strict comparison
fails. Making it non-strict results in treating perfect matches as
good.

1. As getDevice will implicitly call setDevice, in getContext we should not need to call it again.
2. Fix an incorrect type casting.

It was merged by mistake.

This reverts commit ab6be9b7.

Test cases (accuracy and performance) are provided.

It's the same as the implicitly defined one, and it causes a -Wextra warning
(not initializing the base class in a copy constructor).

This has no bearing on compilation, but it makes them show up in IDEs.

receives the number of neighbors joined into each detected object

Also, removing explicit include path configuration,
since QT_USE_FILE takes care of that.

The bug is a buffer overrun when border type is reflect101.
It is found that gfft crashed with input of size 100x100 on Intel CPU.

We don't really need it, it makes the code longer, and it can lead to
inconsistent paths when OpenCV is itself inside a symlink.