Add support for alpha maps to compute_crc32_for_image().

Add support for alpha maps to compute_crc32_for_image().

When a destination image I has an alpha map A, the following rules apply:

   - If I has an alpha channel itself, the content of that channel is
     undefined

   - If A has RGB channels, the content of those channels is
     undefined.

Hence in order to compute the CRC32 for such an image, we have to mask
off the alpha channel of the image, and the RGB channels of the alpha
map.

V2: Shifting by 32 is undefined in C