Multiply `c` by `m` - can adjust for rectangular shaped `c`.

def _affine_mult(c:FlowField,m:AffineMatrix)->FlowField:
    "Multiply `c` by `m` - can adjust for rectangular shaped `c`."
    if m is None: return c
    size = c.flow.size()
    h,w = c.size
    m[0,1] *= h/w
    m[1,0] *= w/h
    c.flow = c.flow.view(-1,2)
    c.flow = torch.addmm(m[:2,2], c.flow,  m[:2,:2].t()).view(size)
    return c