We'll want to use the descriptor type for a helper soon and we don't want
to pull all of view.h with us.

Same as the assign variant, but lets us mutate an existing descriptor
if it exists instead of asserting.

We have to do with for fullscreen surfaces: Fullscreen surfaces
technically aren't part of their container anymore visually (so the
scene doesn't have it) but we want acknowledge the container it belongs
to for input reasons.

This will allow us to simplify node_at_coords later.

…escriptor_find

We can instead search the scene instead of hand rolling this logic