libv4l: Do not set format for control applications

libv4l: Do not set format for control applications

From: Hans de Goede <hdegoede@redhat.com>

When we must do conversion (as we want todo flipping / processing) and
the cam does not default to a supported dest format, we set the emulated
format to rgb24, wait with doing this till the app actually does something
format related, otherwise control applications like v4l2ucp used to
become the stream owner locking out other apps.

Priority: normal

Signed-off-by: Hans de Goede <hdegoede@redhat.com>