Coalesce _xcb_writev into _xcb_out_write and simplify.
Coalesce _xcb_writev into _xcb_out_write and simplify.

OpenInkpot-archive/iplinux-libxcb,OpenInkpot-archive/iplinux-libxcb,stapelberg/libxcb,topcat/xcb-win32,topcat/xcb-win32,topcat/xcb-win32,topcat/xcb-win32,topcat/xcb-win32,stapelberg/libxcb,stapelberg/libxcb,OpenInkpot-archive/iplinux-libxcb