x11: Flag flushes despite having less than the element size

If say we want 32bit data, but have 2 bytes stored, we would simply
ignore flush requests. Allow (and don't clear) the needs_flush flag
if we have less than the element size accumulated.

Instead handle this in can_flush(), so it's triggered whenever we
have enough data to fill 1 element, or if the stream is closing
(seems a broken situation, but triggered by the caller).

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198

(cherry picked from commit 967966cd)