通过创建mask，来实现图片透明渐变效果

通过创建mask，来实现图片透明渐变效果。下面是主要函数，很简单：

void CTestBitmapAppView::CreateMaskBitmap(TRgb aRgb)
{
delete iMarkBg;
iMarkBg = NULL;
iMarkBg = new(ELeave)CFbsBitmap();
iMarkBg->Create(iBg->SizeInPixels(),EGray256);

CFbsBitmapDevice* bitDev = CFbsBitmapDevice::NewL(iMarkBg);
CleanupStack::PushL(bitDev);
CFbsBitGc* bitGc;
bitDev->CreateContext(bitGc);

bitGc->SetPenStyle(CGraphicsContext::ENullPen);
bitGc->SetBrushColor(aRgb);
bitGc->SetBrushStyle(CGraphicsContext::ESolidBrush);
bitGc->DrawRect(TRect(TPoint(0,0),iBg->SizeInPixels()));
bitGc->SetBrushStyle(CGraphicsContext::ENullBrush);
delete bitGc;
CleanupStack::PopAndDestroy(bitDev);

}

void CTestBitmapAppView::Start()
{
if(iMarkRgb.Green() == 0xff)
{
return;
}
iValue+=1;
iMarkRgb.SetBlue(iValue);
iMarkRgb.SetGreen(iValue);
iMarkRgb.SetRed(iValue);
CreateMaskBitmap(iMarkRgb);
DrawDeferred();

iTimer->After(50000);
}