This idiotic change was introduced in stealth mode from 2.76 to 2.77. It has something to do with 3d textures, supposedly. I think the person who made this change is simply an asshat...
Export all images right-side up. OpenGL flips them because fail, so we
have to do a bit of work to correct that. Thankfully, this operation is
not as slow as I thought.
NOTE TO SELF: remember, texture page updating would be nice to have...