Let LineSegmentDetector output line segment coordinates in float precisi...

...on, as supported by the LSD algorithm.

Linked to ticket feature #3890:

Currently, the method LineSegmentDetector::detect returns the detected line segment coordinates with int precision (in a Vec4i object). The LSD algorithm and this implementation internally however work with subpixel precision. Only at the very last moment, the double results are cast to ints.

The type of "lines" output parameter of LineSegmentDetector::detect can be changed from Vec4i to Vec4f.