The number of gaussians involved in a mixture is supposed
to be dynamically adjusted. After being increased, the number
of gaussians can't be reduced anymore.

It seems to be a regression as the legacy code
located in modules/legacy/src/bgfg_gaussmix.cpp allows to reduce
such number of gaussians.

While generating Java JNI wrappers package names with an underscore (`_`) character where not properly escaped according to https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/design.html#resolving_native_method_names (see also: https://github.com/Itseez/opencv_contrib/issues/652).

This fix replaces all the occurrences of `_` with `_1` resulting in proper JNI method names.

update documentation of calcBackProject() calcHist() compareHist()

Enable "useProvidedKeypoints" flag on cuda::ORB's detectAndCompute function in order to able to describe provided keypoints.

* additional author : Ender Tunç <endertunc@yahoo.com>

resolves #6473

 * Enable "useProvidedKeypoints" flag on cuda::ORB's detectAndCompute function in order to able to describe provided keypoints.

   Update for using 'const' reference to avoid array copy on each iteration.

resolves #6473

	Update for 'const' reference to avoid array copy on each iteration.

  * fix issue #6536
  * increase threshold in particular tests due to the difference of fmsub and fsub

  * modify test, too