Aravis SDK: Basic software based autoexposure control

* Basic software based autoexposure control

* Aravis autoexposure: skip frame taken while changing exposure setup

we are drawing on a BGR image declared as RGB, so we should not swap
components here either.

also correctly forward spacing parameter in fontQt

To resolve undefined "Mat_to_vector_KeyPoint" error

Fix findContours crash for very large images (#7451)

* Cast step to size_t in order to avoid integer overflow when processing very large images

* Change assert to CV_Assert