fix for very minor issue with advmame*x
|Reported by:||SF/lord_nightmare||Owned by:||fingolfin|
this fixes an extremely minor issue with advmame2x and
advmame3x, regarding the left screen edge not correctly
assuming that all offscreen pixels are black, which
happens on all three other edges. so this fixes it.
Ticket imported from: #738563. Ticket imported from: patches/329.