Minor optimizations in drawing rects

ponydos.asm

@@ -156,26 +156,20 @@ draw_rect:
 	shl cx, 1
 
 	.loop:
-		; Any rows left to copy?
-		cmp dx, 0
-		je .end
-		dec dx
-
 		; Copy a row
-		push cx
+		pusha
 		rep movsb
-		pop cx
+		popa
 
 		; Move to the next row in the input buffer
 		add si, bx
-		sub si, cx
 
 		; Move to the next row in the screen buffer
 		add di, 2*COLUMNS
-		sub di, cx
 
-		jmp .loop
-	.end:
+		dec dx
+		; Any rows left to copy?
+		jnz .loop
 
 	pop es
 	popa