Add facilities in main() to allow selection and usage of jpg_source.

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Now that we have test-source which provides video by generating a test
pattern in userspace, allow its selection in the command line options.
Since it isn't backed by anything, it is set as the default video source
if nothing else is specified.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Provide infrastructure to be able to build the uvc-gadget library as a
shared object and an executable which links against this as a dependency
to implment the reference 'uvc-gadget' application.

All existing cmake infrastructure is removed. This removes custom glob
support that was previously used by an early development for Android,
which is expected to be re-added later when required.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Bringing in the meson infrastructure will enable extra warnings,
including -Wunused...

Directly reference the two locations that have known unused arguments.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Move the main uvc-gadget application to src/ to make it clear
there is a separation between library and application code.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>