Trying to get rid of gdi.h

[Rémi Assailly]

Hi all

I started a work to get rid of GDI_GetObjPtr and GDI_ReleaseObj in 
x11drv but I am absolutely not sure if I have chosen a good way.
What I send you is incomplete : some GDI_* are still staying and we 
cannot get rid of those functions using my way.

After getting rid of GDI_* we'll be able to get rid of BITMAPOBJ* and 
PALETTEOBJ* using opaque pointers as Alexandre told me.

Rémi