wayland: don't prematurely release EGL buffers

commit 0165cb6974 changed
mutter to release committed shm buffers as soon as they were
uploaded to the GPU.

It also inadvertently changed mutter to prematurely
release EGL buffers (which never get copied, but get used
directly).

This commit corrects that mistake.

https://bugzilla.gnome.org/show_bug.cgi?id=761312

src/wayland/meta-wayland-buffer.c

@@ -140,6 +140,9 @@ meta_wayland_buffer_ensure_texture (MetaWaylandBuffer *buffer)
 
   buffer->texture = texture;
 
+  if (shm_buffer)
+    buffer->copied_data = TRUE;
+
  out:
   return buffer->texture;
 }