libweston: Rename weston_surface_destroy() to weston_surface_unref()

Make it obvious that weston_surface has a reference counting happening
and destruction of the weston_surface happens when the last
weston_surface reference has been accounted for.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

tests/surface-global-test.c

@@ -90,5 +90,5 @@ PLUGIN_TEST(surface_to_from_global)
 	assert(ix == 0 && iy == 0);
 
 	/* Destroys all views too. */
-	weston_surface_destroy(surface);
+	weston_surface_unref(surface);
 }

tests/surface-test.c

@@ -70,5 +70,5 @@ PLUGIN_TEST(surface_transform)
 	assert(x == 200 && y == 340);
 
 	/* Destroys all views too. */
-	weston_surface_destroy(surface);
+	weston_surface_unref(surface);
 }