client: Add mediated reader stopping for display proxy
Add display API for negotiated display event reader's early exits.
This patch allows display event readers to exit their polling and processing loops early based on when something calls wl_display_stop.
TODO: There is still a potential issue of new threads starting after wl_display_stop has returned, which would need a more significant change to give the display API control of the display pointer, and check whether it is nulled before dereferencing throughout the current API.
Signed-off-by: Robert Beckett email@example.com