Overhaul the Xlib winsys layer.

Overhaul the Xlib winsys layer.

Front/back color buffers are now allocated with ordinary malloc() via the
winsys buffer functions.
To display surfaces in SwapBuffers() or flush_frontbuffer() we create an
XImage that wraps the surface, then use XPutImage to copy to the window.
Shared memory transport disabled for now.