set sel.alt in selstart instead of selextend

st.c

@@ -446,6 +446,7 @@ selstart(int col, int row, int snap)
 	selclear();
 	sel.mode = SEL_EMPTY;
 	sel.type = SEL_REGULAR;
+	sel.alt = IS_SET(MODE_ALTSCREEN);
 	sel.snap = snap;
 	sel.oe.x = sel.ob.x = col;
 	sel.oe.y = sel.ob.y = row;
@@ -474,7 +475,6 @@ selextend(int col, int row, int type, int done)
 	oldsey = sel.ne.y;
 	oldtype = sel.type;
 
-	sel.alt = IS_SET(MODE_ALTSCREEN);
 	sel.oe.x = col;
 	sel.oe.y = row;
 	selnormalize();