v4l2object: use G_SELECTION instead of G_CROP in gst_v4l2_object_acquire_format

v4l2object: use G_SELECTION instead of G_CROP in gst_v4l2_object_acquire_format

The gst_v4l2_object_acquire_format() function is used by v4l2videodec to obtain
the currently set capture format. Since G_FMT returns the coded size, the
visible size needs to be obtained from the compose rectangle in order to
negotiate it with downstream elements. The G_CROP call hasn't worked on mem2mem
capture queues for a long time. Instead use the G_SELECTION call to obtain the
compose rectangle and only fall back to G_CROP for ancient kernels.

https://bugzilla.gnome.org/show_bug.cgi?id=766381