rnnca: add option to set the input offsets

Syntax is (as before):

    pair := [0-9][0-9]

    offsets := 'Y' pair+ 'C' pair+

Each pair is an offset from the origin which is reflected 8 ways. For
example '12' means the points at 1,2; 1,-2; -1,2; -1,-2; 2,1; 2,-1;
-2,1; and -2,-1. The coordinate pairs following 'Y' are for the luma
plane, and those following 'C' are for the chroma planes.