x11: don't block in buffer acquire

x11: don't block in buffer acquire

Don't ever block when acquiring a buffer from the bufferpool in the fallback
mode. If we block, we might deadlock when going to PAUSED because we never
unlock when going to paused.

The acquire can block when there are no more buffers in the pool, this is a
sign that the pool is too small. Since we are the only ones using the pool in
the fallback case and because we scale the buffer, someone else must be using
our pool as well and is doing something bad.