Update the SLA slices at the print preview once the index is calculated.

src/libslic3r/PrintBase.hpp

@@ -272,6 +272,7 @@ public:
             NO_RELOAD_SCENE                 = 0,
             RELOAD_SCENE                    = 1 << 1,
             RELOAD_SLA_SUPPORT_POINTS       = 1 << 2,
+            RELOAD_SLA_PREVIEW              = 1 << 3,
         };
         // Bitmap of FlagBits
         unsigned int    flags;