ImGuiListClipper: internal rework and tidying up to facilitate supporting frozen rows in tables + stop promoting using constructors parameters.

imgui.h

@@ -1900,7 +1900,8 @@ struct ImGuiStorage
 // - Step 3: the clipper validate that we have reached the expected Y position (corresponding to element DisplayEnd), advance the cursor to the end of the list and then returns 'false' to end the loop.
 struct ImGuiListClipper
 {
-    int     DisplayStart, DisplayEnd;
+    int     DisplayStart;
+    int     DisplayEnd;
     int     ItemsCount;
 
     // [Internal]
@@ -1911,12 +1912,12 @@ struct ImGuiListClipper
     // items_count:  Use -1 to ignore (you can call Begin later). Use INT_MAX if you don't know how many items you have (in which case the cursor won't be advanced in the final step).
     // items_height: Use -1.0f to be calculated automatically on first step. Otherwise pass in the distance between your items, typically GetTextLineHeightWithSpacing() or GetFrameHeightWithSpacing().
     // If you don't specify an items_height, you NEED to call Step(). If you specify items_height you may call the old Begin()/End() api directly, but prefer calling Step().
-    ImGuiListClipper(int items_count = -1, float items_height = -1.0f)  { Begin(items_count, items_height); } // NB: Begin() initialize every fields (as we allow user to call Begin/End multiple times on a same instance if they want).
-    ~ImGuiListClipper()                                                 { IM_ASSERT(ItemsCount == -1); }      // Assert if user forgot to call End() or Step() until false.
+    ImGuiListClipper(int items_count = -1, float items_height = -1.0f);
+    ~ImGuiListClipper();
 
-    IMGUI_API bool Step();                                              // Call until it returns false. The DisplayStart/DisplayEnd fields will be set and you can process/draw those items.
     IMGUI_API void Begin(int items_count, float items_height = -1.0f);  // Automatically called by constructor if you passed 'items_count' or by Step() in Step 1.
     IMGUI_API void End();                                               // Automatically called on the last call of Step() that returns false.
+    IMGUI_API bool Step();                                              // Call until it returns false. The DisplayStart/DisplayEnd fields will be set and you can process/draw those items.
 };
 
 // Helpers macros to generate 32-bit encoded colors