v4l2src: Reset the compose window to the default after setting format

v4l2src: Reset the compose window to the default after setting format

When the size of V4L2 capture or output is changes with VIDIOC_S_FMT,
the device is only required to update the compisition window to fit
inside the new frame size.  This can result in captured data only being
updated on a portion of the frame after a resize.

Update the composition window to the default value determined by the
V4L2 device driver whenever the format is changed to make sure that
all image data is composed to its full size.

Fixes #765

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1806>