GDI+ Fenster flackert

#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
#include <GDIPlus.au3>

Global $width = 302
Global $height = 302

$hGUI = GUICreate("BlueJ Figuren Demo", $width, $height)
$hWnd = WinGetHandle($hGUI)
;~ GUISetBkColor(0, $hGUI)
;~ GUISetBkColor(0xFFFFFF, $hGUI)
;~ WinSetTrans($hGUI, "", 255)
GUISetState(@SW_SHOW)

_GDIPlus_Startup()
$hGraphic = _GDIPlus_GraphicsCreateFromHWND($hWnd)
$Bitmap = _GDIPlus_BitmapCreateFromGraphics($width, $height, $hGraphic) ;create bitmap
$backbuffer = _GDIPlus_ImageGetGraphicsContext($bitmap)
_GDIPlus_GraphicsSetSmoothingMode($hGraphic, 2)
$hBrush = _GDIPlus_BrushCreateSolid(0xFFFFFF00)

_GDIPlus_GraphicsClear($hGraphic)
_GDIPlus_GraphicsFillPie($hGraphic, $width / 2 - 60 / 2, $height / 2 - 60 / 2, 60, 60, 1, 360, $hBrush)

Local $a = 1

While GUIGetMsg() <> $GUI_EVENT_CLOSE
_GDIPlus_GraphicsClear($backbuffer)
_GDIPlus_GraphicsClear($backbuffer)
_GDIPlus_GraphicsFillPie($backbuffer, $a, 50, 60, 60, 1, 360, $hBrush)
_GDIPlus_GraphicsDrawImageRect($hgraphic, $bitmap, 0, 0, $width, $height)

$a += 1
Sleep(10)
WEnd