Create standalone public wayland client/server headers

Because the wayland-client-protocol.h header defines symbols that
collide with the wayland-server-protocol.h header we allow applications
to explicitly ensure that they are only including one at a time by
exposing corresponding <cogl/cogl-wayland-client.h> and
<cogl/cogl-wayland-server.h> headers. This also adds a missing guard to
cogl-texture-2d.h that it isn't included directly.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

examples/cogland.c

@@ -1,4 +1,5 @@
 #include <cogl/cogl.h>
+#include <cogl/cogl-wayland-server.h>
 #include <glib.h>
 #include <stdio.h>
 #include <sys/time.h>