wayland: Rename DoubleBufferedState to PendingState

I was talking with other people and they became confused at the
term "double-buffered", since we were also talking about
double-buffering in general, e.g. swapping between two buffers.

Instead, we'll adapt the "pending state" nomenclature that we
already use for the field / variable names.
wayland: Remove an extraneous variable
wayland: Rename pending_surface_state => pending

This matches what's normally done.