Make Show2 hide its window if finalize is called, fix a video reading problem

Make show2 hide its window if finalize is called, also change the update image
signal to a blocking type to allow this to work correctly (this will actually
lock the frame rate of a stream to whatever speed qt can update windows at, so
maybe should be optional)

After calling VideoCapture::read, make a clone of the returned matrix since it
has become clear that in at least some cases, the returned matrix is an alias
of some internal buffer of the VideoCapture.