It's not an offset from pixmap coords to composited pixmap coords,
it's an offset from screen-relative window drawable coords to
composited pixmap coords.
Here's my theory for the flipping: It doesn't really matter which
orientation we store the pixmaps if we don't scan them out. We have
to flip coordinates for the window system framebuffer. Doing so for
everything else makes things consistent. I'm not sure how this will
interact with future GLX integration, though.
