wayland: Don't leak the existing texture if we already have one

We were missing a check in ensure_buffer_texture that checked if we
already had a CoglTexture bound for the buffer.

src/wayland/meta-wayland-surface.c

@@ -151,6 +151,9 @@ ensure_buffer_texture (MetaWaylandBuffer *buffer)
   CoglError *catch_error = NULL;
   CoglTexture *texture;
 
+  if (buffer->texture)
+    return;
+
   texture = COGL_TEXTURE (cogl_wayland_texture_2d_new_from_buffer (ctx,
                                                                    buffer->resource,
                                                                    &catch_error));