Fix depth offsets for sprite with non-zero ZRamp.

Bibs and other effects that should be drawn at ground level
can now simply define ZRamp: 1, Offset: <X>,<Y>,1, avoiding the
need to account for the Y offset or internal sprite offsets.

mods/ts/sequences/vehicles.yaml

@@ -22,7 +22,7 @@ harv.gdi:
 	harvest: harvestr
 		Length: *
 		ZRamp: 1
-		Offset: 0, 0, 3
+		Offset: 0, 0, 1
 	icon: sidebar-gdi|harvicon
 
 harv.nod:
@@ -30,7 +30,7 @@ harv.nod:
 	harvest: harvestr
 		Length: *
 		ZRamp: 1
-		Offset: 0, 0, 3
+		Offset: 0, 0, 1
 	icon: sidebar-nod|harvicon
 
 hvr: