vout: remove Thread and ThreadDisplay prefixes

Some static functions were named with prefixes. For unknown reasons, there were two different prefixes (Thread and ThreadDisplay).

Their usage is confusing, because these functions could be called either from Thread() (the vout thread) or from another thread. For example, vout_NextPicture() is called from the decoder thread, and it calls ThreadDisplayPicture(), which itself calls many of the other prefixed functions.

To avoid confusion (and for simplicity), remove these prefixes.