Add new test of filter reduction from BILINEAR to NEAREST

This new test tests a bunch of bilinear downscalings, where many have
a transformation such that the BILINEAR filter can be reduced to
NEAREST (and many don't).

A CRC32 is computed for all the resulting images and compared to a
known-good value for both 4-bit and 7-bit interpolation.

V2: Remove leftover comment, some minor formatting fixes, use a
timestamp as the PRNG seed.

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