xfreerdp: fix RDP order color conversion

Note: /gdi:sw was working fine, this commit fixes /gdi:hw

* calculate color channel shifts based on X11 visual color masks
* fast path to skip conversion if visual color masks equal rdp color masks
* successfully tested 8/15/16/24/32 bpp rdp sessions on 16/24/32 bpp visuals
Merge pull request #2277 from nfedera/fix-2014-12-15-01

xfreerdp: fix RDP order color conversion