wayland/buffer: use G_UNLIKELY for error paths
We do a lot of if-checks in MetaWaylandBuffer
to deal with unexpected
situations (as we should). However, that means we get a branching
penalty for every time this leads to a misprediction. Although these
should in theory be mitigated by a proper branch predictor, it doesn't
hurt giving some hints to make sure that we don't sacrifice our hot
path. Especially given that in the case of video streams, the
meta_wayland_buffer_attach
is called for example 60 times per second.