Another wl_display_read_events bug

[Marek Chalupa]

Hi,

I discovered another bug in wl_display_read_events(). In this series
I'm adding a test and fix for it. Hopefully, these are the last patches
regarding threading from me atm (except for the promised documentation).

  /* cosmetic changes and code reuse */
  [PATCH 1/4] client: add display_wakeup_threads function
  [PATCH 2/4] display-test: make use of create_thread function

  /* test and fix itselfs */
  [PATCH 3/4] tests: add display test that highlights another bug in
  [PATCH 4/4] client: fix bug with display->reader_count < 0

  Cheers,
  Marek