Fix m_rasterColourOp uninitialized value

The problem is reported by valgrind in ApplyRasterColourOp.
Setting it to wxQtNONE avoids the report but doesn't fix some drawing issues (don't know if it is related), so more investigation is needed about the initial value to get a correct behavior (if it should be used at all without calling first SetLogicalFunction)
Add dtor for wxBitmapRefData to fix memory leak in wxQT