0021981: Additional callback before redraw procedure

src/Aspect/Aspect_GraphicCallbackProc.hxx

@@ -15,13 +15,20 @@
 #define OCC_REDRAW_WINDOWAREA 2
 #define OCC_REDRAW_BITMAP 3
 
-/* 
-   This flag, when bitwise OR`ed with the "reason" value, 
-   informs the callback that it is called before redrawing the overlayer .
-   Otherwise, the callback is invoked after the overlayer is redrawn.        
-*/
+// The flags below provide additional information to define the moment when
+// callback was invoked in redraw procedure. These flags are bitwise OR'ed
+// with the "reason" value of callback:
+// 1) OCC_PRE_REDRAW  - callback was invoked before redrawing underlayer
+//                      ( at the beginning of redraw procedure );
+// 2) OCC_PRE_OVERLAY - callback was invoked before redrawing overlayer;
+// Otherwise, if no flags added to the "reason" value, the callback was
+// invoked at the end of redraw ( after the overlayer is redrawn )
+#define OCC_PRE_REDRAW  0x4000
 #define OCC_PRE_OVERLAY 0x8000
 
+// mask for all additional callbacks that invoked in process of redrawing
+#define OCC_REDRAW_ADDITIONAL_CALLBACKS ( OCC_PRE_REDRAW | OCC_PRE_OVERLAY )
+
  typedef struct {
    int reason;
    int wsID;