present: Handle event mask updates as specified v2

From the Present extension specification:

 An event context is associated with a specific window; using
 an existing event context with a different window generates
 a Match error.

 If eventContext specifies an existing event context, then if
 eventMask is empty, PresentSelectInput deletes the specified
 context, otherwise the specified event context is changed to
 select a different set of events.

 If eventContext is an unused XID, then if eventMask is empty
 no operation is performed. Otherwise, a new event context is
 created selecting the specified events.

Without this change, there's no way for a client to explicitly change
or destroy an existing event mask entry. Trying to do so as specified
above would just result in a protocol error.

v2: (Keith Packard)
* Use dixLookupResourceByType instead of walking window_priv->events
* Return BadMatch if the existing event context is associated with a
  different window or client
* Call LEGAL_NEW_RESOURCE again when creating a new event context
* Drop invalid "leak fix"

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

present/present_request.c

@@ -184,8 +184,6 @@ proc_present_select_input (ClientPtr client)
 
     REQUEST_SIZE_MATCH(xPresentSelectInputReq);
 
-    LEGAL_NEW_RESOURCE(stuff->eid, client);
-
     rc = dixLookupWindow(&window, stuff->window, client, DixGetAttrAccess);
     if (rc != Success)
         return rc;