Fix wxGenericColourButton bitmaps for different button states

Only the bitmap of the normal state was updated, not of the other (focused, hover, disabled, etc) states.
This became obvious after ee93f4cae8, but already happened before that for the disabled state.

Invalidate the bitmapButton using wxNullBitmap, so all states will be updated when setting the bitmap.

src/generic/clrpickerg.cpp

@@ -149,6 +149,8 @@ void wxGenericColourButton::UpdateColour()
     }
 
     dc.SelectObject( wxNullBitmap );
+
+    SetBitmapLabel( wxNullBitmap );
     SetBitmapLabel( m_bitmap );
 }