Fixed issue 3130 (http://code.opencv.org/issues/3130), where one argument of the BRISK-call was ignored. Previously it was not possible to use BRISK without creating descriptors.
Now it behaves like ORB (and how it is documented), and you can call BRISK to just generate feature points and no descriptors.

Previously the function requires Info::Impl implicitly. A call will create
new Context this function which is not intended.
The properties are now moved to global scope to fix this issue.