fix picture jumping to incorrect X position after dragging

If a picture anchored to character is dragged vertically, it can actually
jump to the left, even though X should stay the same. This change
basically reverts part of 5ddd93f4f90b3627a0ba63b62a5367c5e6c112a1.
I haven't figured out how this GetBaseOfstForFly() actually works, but
I'm reasonably sure that the relative X offset introduced by it
in SwFlyAtCntFrm::SetAbsPos() is not undone anywhere in SwFlyFrm::ChgRelPos()
or otherwise acted upon, it simply causes relative X position change, even though
the absolute position remained the same.

Change-Id: Iabf4f4e4268711a36f02aa47ca9624f9007a22a6