Fix IdxBuffer related comments. (#4845, #4863)

imgui.h

@@ -2278,16 +2278,16 @@ typedef void (*ImDrawCallback)(const ImDrawList* parent_list, const ImDrawCmd* c
 #define ImDrawCallback_ResetRenderState     (ImDrawCallback)(-1)
 
 // Typically, 1 command = 1 GPU draw call (unless command is a callback)
-// - VtxOffset/IdxOffset: When 'io.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset' is enabled,
-//   those fields allow us to render meshes larger than 64K vertices while keeping 16-bit indices.
-//   Pre-1.71 backends will typically ignore the VtxOffset/IdxOffset fields.
+// - VtxOffset: When 'io.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset' is enabled,
+//   this fields allow us to render meshes larger than 64K vertices while keeping 16-bit indices.
+//   Backends made for <1.71. will typically ignore the VtxOffset fields.
 // - The ClipRect/TextureId/VtxOffset fields must be contiguous as we memcmp() them together (this is asserted for).
 struct ImDrawCmd
 {
     ImVec4          ClipRect;           // 4*4  // Clipping rectangle (x1, y1, x2, y2). Subtract ImDrawData->DisplayPos to get clipping rectangle in "viewport" coordinates
     ImTextureID     TextureId;          // 4-8  // User-provided texture ID. Set by user in ImfontAtlas::SetTexID() for fonts or passed to Image*() functions. Ignore if never using images or multiple fonts atlas.
     unsigned int    VtxOffset;          // 4    // Start offset in vertex buffer. ImGuiBackendFlags_RendererHasVtxOffset: always 0, otherwise may be >0 to support meshes larger than 64K vertices with 16-bit indices.
-    unsigned int    IdxOffset;          // 4    // Start offset in index buffer. Always equal to sum of ElemCount drawn so far.
+    unsigned int    IdxOffset;          // 4    // Start offset in index buffer.
     unsigned int    ElemCount;          // 4    // Number of indices (multiple of 3) to be rendered as triangles. Vertices are stored in the callee ImDrawList's vtx_buffer[] array, indices in idx_buffer[].
     ImDrawCallback  UserCallback;       // 4-8  // If != NULL, call the function instead of rendering the vertices. clip_rect and texture_id will be set normally.
     void*           UserCallbackData;   // 4-8  // The draw callback code can access this.