And: Bug 593251 - Mask positions may be incorrect when opening a PSD

The position-relative flag of layer masks doesn't specify how the
mask position is represented in the PSD -- it's always absolute --
but rather whether the layer and the mask are linked or unlinked.
Since masks are always linked to their layers in GIMP, just ignore
this flag (and, when saving, clear the flag so that the mask is
linked to layer when loaded in PS.)