libv4l: add support for adding black borders (reverse cropping)

libv4l: add support for adding black borders (reverse cropping)

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

Add the capability to provide 320x240 to apps if the cam can only
do 320x232 (some zc3xx cams) by adding black borders. And more in
general the capability to make certain standard resolutions available
by adding black borders to slightly smaller resolutions, in case we
encounter more cams which have a hardware limitation which makes them do
a resolution slightly smaller then the standard ones.

Priority: normal

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