Fix some build warnings

private field 'm_dwCookie' is not used
'return' will never be executed
result of comparison of unsigned enum expression < 0 is always false
'FlushDC' overrides a member function but is not marked 'override'
potentially uninitialized local variable 'bound' used

tests/graphics/affinematrix.cpp

@@ -311,7 +311,7 @@ public:
         m_gcdc->SetGraphicsContext(ctx);
     }
 
-    virtual void FlushDC()
+    virtual void FlushDC() wxOVERRIDE
     {
         // Apparently, flushing native Direct2D renderer
         // is not enough to update underlying DC (bitmap)