wlr_scene: Call surface enter scale events on surface create

When we create a buffer with wlr_scene_buffer_create, it will immediately add itself to the scene, this means that it also immediately will try to compute surface events. Because we have no chance to add our signals to the buffer on create of the buffer node itself, we miss all these events.

To fix this, let's add a helper that allows a scene buffer to be created with the default state disabled to which we can enable it later and catch all the output enter events.

A cleaner solution would be to allow scene nodes to have a NULL parent so that we can initially create the buffer node with a null parent, then re-parent it once we're read. This is a lot more involved and outside of the scope of a fix.