gtk/unx: remove successive calls to g_main_context_iteration

Replace the sequence:
    non-blocking call to g_main_context_iteration
    if no-event-recvd
        blocking call to g_main_context_iteration
With this one:
    blocking call to g_main_context_iteration until first evt reveived.
This allows more longer sleep period, when nothing is going on (verified
with strace).

Change-Id: I8f4ea911534c90aea091fd68e9f09db11591392f
Reviewed-on: https://gerrit.libreoffice.org/4777Reviewed-by: Petr Mladek <pmladek@suse.cz>
Tested-by: Petr Mladek <pmladek@suse.cz>

vcl/unx/gtk/app/gtkdata.cxx

@@ -571,12 +571,10 @@ void GtkData::Yield( bool bWait, bool bHandleAllCurrentEvents )
             gboolean wasOneEvent = TRUE;
             while( nMaxEvents-- && wasOneEvent )
             {
-                wasOneEvent = g_main_context_iteration( NULL, FALSE );
+                wasOneEvent = g_main_context_iteration( NULL, bWait & !bWasEvent );
                 if( wasOneEvent )
                     bWasEvent = true;
             }
-            if( bWait && ! bWasEvent )
-                bWasEvent = g_main_context_iteration( NULL, TRUE ) != 0;
         }
         else if( bWait )
         {