st/egl: Reduce validation round-trips in ximage backend.

st/egl: Reduce validation round-trips in ximage backend.

ximage_surface_validate is called several times per frame.  This commit
adds the client and server stamps to reduce the round-trips to the
server.  The idea is to bump the server stamp when flush_frontbuffer or
swap_buffers is called, and to skip the round-trip when the client stamp
is equal to the server stamp.  This makes sure the client APIs get the
new buffers when a new frame is started while skipping all round-trips
during the drawing.  To make this work, egl_g3d_validate_context is no
longer called after swap_buffers.