pan/blit: Fix 3D blittering

pan/blit: Fix 3D blittering

Fixes several problems in the pan_blit() logic:

1. We actually need the reciprocal of the depth scaling in z_scale (maybe
   we should rename this field z_scale_rcp to make it clear)

2. When Z end < Z start we should remove one to the cur_layer/layer_offset
   instead of doing it on the last_layer field, otherwise there's an
   off-by-one error

3. The Z src offset should be adjusted to account for scaling. If we don't
   do that we won't sample from the right layer when upscaling.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12961>