We need to acquire the yield mutex here

Otherwise we get an assertion failure in a dbgutil build from the
DBG_TESTSOLARMUTEX() in OutputDevice::ImplReleaseGraphics() when
quitting, at least in 64-bit LO on OS X.

Change-Id: Ieb54e6b58033279f807fe91f4f6139018f643903

vcl/source/gdi/virdev.cxx

@@ -184,6 +184,10 @@ VirtualDevice::~VirtualDevice()
 
     ImplSVData* pSVData = ImplGetSVData();
 
+    // OutputDevice::ImplReleaseGraphics() requires the yield mutex to
+    // be held, hmm.
+    pSVData->mpDefInst->AcquireYieldMutex( 1 );
+
     ImplReleaseGraphics();
 
     if ( mpVirDev )