MjpegServer uses the timeout to generate keep-alives to any clients
(which helps detect disconnects and avoid stale client threads).
CvSink GrabFrame now defaults to a timeout, but the timeout can be
changed by the user, or the old no-timeout version is now available
as GrabFrameNoTimeout.
This makes them available for both UsbCamera and HttpCamera / AxisCamera.
To avoid virtual functions in the public-facing interface, move the
implementation of the camera settings functions to the core library.
The code now automatically resizes as required.
This change also disconnects camera resolution settings from MJPEG
stream connections; setting the camera resolution can now only be done
via code.
For functions where a SmallVector is passed to be used as a stack buffer for
the return value, have the return value be the appropriate StringRef or
ArrayRef type. This allows for both more natural usage and enables directly
returning (rather than copying) a permanently stored or constant string.
