More general BILINEAR=>NEAREST reduction

Generalize and simplify the code that reduces BILINEAR to NEAREST so
that the reduction happens for all affine transformations where
t00...t12 are integers and (t00 + t01) and (t10 + t11) are both
odd. This is a sufficient condition for the resulting transformed
coordinates to be exactly at the center of a pixel so that BILINEAR
becomes identical to NEAREST.

V2: Address some comments by Bill Spitzak

Signed-off-by: Søren Sandmann <soren.sandmann@gmail.com>
Reviewed-by: Bill Spitzak <spitzak@gmail.com>