[Gtk2] Alpha blending on GTK2 under Linux

[Valdas Jankūnas]

I working with alpha blending. On QT it works fine. On GTK2 I get 
"Depth: 24" when executing "GetDescriptionFromDevice(0)" (on QT -> 32). 
Same situation in Raspbian Jessie (2017-04-10). Why GTK2 "screen" not 
supports alpha?
  It is strange because if I load PNG with transparency (image with 
gradient) into TImage then I clearly can see alpha blending effect on 
GTK2 when image is displayed. Maybe I must to use 
"GetDescriptionFromDevice" not from screen? From where then?

There is full "DataDescription" on GTK2:
            Format: ricfRGBA
             Width: 3
            Height: 3
             Depth: 24
          BitOrder: riboBitsInOrder
         ByteOrder: riboLSBFirst
         LineOrder: riloTopToBottom
           LineEnd: rileDWordBoundary
      BitsPerPixel: 32
           RedPrec: 8
          RedShift: 16
         GreenPrec: 8
        GreenShift: 8
          BluePrec: 8
         BlueShift: 0
         AlphaPrec: 0
        AlphaShift: 0
  MaskBitsPerPixel: 1
PaletteColorCount: 0

System: Kubuntu 16.10; lazarus v1.9.0; fpc v3.0.0; gtk2 v2.24.30

P.S. sorry for my bad English.

-- 
   Valdas Jankūnas