X11: Convert GDI color to X11 color troughout.
* client/X11/xf_client.c (xf_pre_connect): Set the field `colormap' of
`struct xfContext' to the default colormap.
* client/X11/xfreerdp.h (xf_gdi_get_color): Declare new external function.
* client/X11/xf_gdi.c (xf_gdi_get_color): Define new external function.
Use xf_gdi_get_color() to get the appropriate X color index from the GDI
color representation.
* client/X11/xf_graphics.c: Likewise.