Clipper: added SeekCursorForItem() function, for use when using ImGuiListClipper::Begin(INT_MAX). (#1311)

Tagging #3609 just in case we made a mistake introducing a regression (but tests are passing and have been extended).

imgui.h

@@ -28,7 +28,7 @@
 // Library Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals, e.g. '#if IMGUI_VERSION_NUM >= 12345')
 #define IMGUI_VERSION       "1.91.0 WIP"
-#define IMGUI_VERSION_NUM   19094
+#define IMGUI_VERSION_NUM   19095
 #define IMGUI_HAS_TABLE
 
 /*
@@ -2601,9 +2601,10 @@ struct ImGuiListClipper
     int             ItemsCount;         // [Internal] Number of items
     float           ItemsHeight;        // [Internal] Height of item after a first step and item submission can calculate it
     float           StartPosY;          // [Internal] Cursor position at the time of Begin() or after table frozen rows are all processed
+    double          StartSeekOffsetY;   // [Internal] Account for frozen rows in a table and initial loss of precision in very large windows.
     void*           TempData;           // [Internal] Internal data
 
-    // items_count: 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_count: 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, and you can call SeekCursorForItem() manually if you need)
     // items_height: Use -1.0f to be calculated automatically on first step. Otherwise pass in the distance between your items, typically GetTextLineHeightWithSpacing() or GetFrameHeightWithSpacing().
     IMGUI_API ImGuiListClipper();
     IMGUI_API ~ImGuiListClipper();
@@ -2616,6 +2617,11 @@ struct ImGuiListClipper
     inline void     IncludeItemByIndex(int item_index)                  { IncludeItemsByIndex(item_index, item_index + 1); }
     IMGUI_API void  IncludeItemsByIndex(int item_begin, int item_end);  // item_end is exclusive e.g. use (42, 42+1) to make item 42 never clipped.
 
+    // Seek cursor toward given item. This is automatically called while stepping.
+    // - The only reason to call this is: you can use ImGuiListClipper::Begin(INT_MAX) if you don't know item count ahead of time.
+    // - In this case, after all steps are done, you'll want to call SeekCursorForItem(item_count).
+    IMGUI_API void  SeekCursorForItem(int item_index);
+
 #ifndef IMGUI_DISABLE_OBSOLETE_FUNCTIONS
     inline void IncludeRangeByIndices(int item_begin, int item_end)      { IncludeItemsByIndex(item_begin, item_end); } // [renamed in 1.89.9]
     inline void ForceDisplayRangeByIndices(int item_begin, int item_end) { IncludeItemsByIndex(item_begin, item_end); } // [renamed in 1.89.6]