Add stride option to override the padding added by the driver

V4L2 allows specifying extra padding at the end of a line of pixels, the driver can also override this to have a higher minimum padding than 0.

This adds a libdng_set_stride(dng, stride) function to override the minimum padding that normally gets calculated by libdng when packing the data.

The makedng utility now has a -S option to specify the stride in bytes for a single row of pixels.