Fix bug in copy image mipmap comparisons

[joshqti]

The destination image for mipmap copies is incorrect, use the width of the mip-level instead of mip-level 0. The check if (where < dstImageInfo->width) is not correct, width is the size of level 0, but it should be the width of the destination mip level.

Deletes redundant scanline comparison, which is confusing and uneccesary. compare_scanline and memcmp do the same thing, except compare_scanline correctly take image format into account, whereas memcmp finds all differences.