TINSEL: Fix inventory scrollbar handle position

The scrollbar handle was one pixel too far to the left, and the
topmost position was off by one as well. I've verified this to the
best of my ability. I've compared it visually in DOSBox (which is
a bit tricky for Discworld 2, but I think it's right now), and
against the initial Tinsel v1 source code that was added to
ScummVM. I don't know what the initial Tinsel v2 source code
looked like, though.

engines/tinsel/dialogs.cpp

@@ -2723,7 +2723,7 @@ static OBJECT *AddObject(const FREEL *pfreel, int num) {
 
 static void AddSlider(OBJECT **slide, const FILM *pfilm) {
 	g_SlideObject = *slide = AddObject(&pfilm->reels[IX_SLIDE], -1);
-	MultiSetAniXY(*slide, MultiRightmost(g_RectObject) + (TinselV2 ? NM_SLX : -M_SXOFF + 2) - 1,
+	MultiSetAniXY(*slide, MultiRightmost(g_RectObject) + (TinselV2 ? NM_SLX : -M_SXOFF + 2),
 		g_InvD[g_ino].inventoryY + g_sliderYpos);
 	MultiSetZPosition(*slide, Z_INV_MFRAME);
 }
@@ -3318,7 +3318,7 @@ static void ConstructInventory(InventoryType filling) {
 				}
 			}
 		} else if (g_InvD[g_ino].NoofItems > g_InvD[g_ino].NoofHicons*g_InvD[g_ino].NoofVicons) {
-			g_sliderYmin = g_TLheight - (TinselV2 ? 2 : 1);
+			g_sliderYmin = g_TLheight - (TinselV2 ? 1 : 2);
 			g_sliderYmax = g_TLheight + eV + (TinselV2 ? 12 : 10);
 			AddSlider(&retObj[n++], pfilm);
 		}