test: Add new alphamap test program.

test: Add new alphamap test program.

This program demonstrates three bugs relating to alpha maps:

- When fetching from an alpha map into 32 bit intermediates, we use
  the fetcher from the image, and not the one from the alpha map.

- For 64 bit intermediates we call fetch_pixel_generic_lossy_32()
  which then calls fetch_pixel_raw_64, which is NULL because alpha
  images are never validated.

- The alpha map should be used *in place* of any existing alpha
  channel, but we are actually multiplying it onto the image.