Fix wxOSX wxDataViewRenderers' alignment handling

Don't override renderer's wxDVR_DEFAULT_ALIGNMENT alignment when
column's alignment is set. Call a method to recompute effective
alignment instead.

Related to a6be5bda that fixed a similar issue in wxGTK.

include/wx/osx/dvrenderer.h

@@ -76,6 +76,7 @@ public:
 
     void SetNativeData(wxDataViewRendererNativeData* newNativeDataPtr);
 
+    void OSXUpdateAlignment();
 
 #if wxOSX_USE_COCOA
     // called when a value was edited by user