gdiplus: partially implement GdipBitmapSetPixel

[Nikolay Sivov]

Massimo Del Fedele wrote:
> This one implements GdipBitmapSetPixel() besides unhandled alpha channel.
>
> Ciao
>
> Max

+    SetPixel(hdc, x, y, ((color << 16) & 0xff0000) | (color & 0x00ff00) | ((color >> 16) & 0x0000ff));

Use ARGB2COLORREF(ARGB color) from gdiplus.c for that.