ImDrawList: Store header/current ImDrawCmd in instance to simplify merging code. Amend 0320e72, toward #3163, #3129

imgui.h

@@ -1874,19 +1874,21 @@ 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)
-// Pre 1.71 back-ends will typically ignore the VtxOffset/IdxOffset fields. When 'io.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset'
-// is enabled, those fields allow us to render meshes larger than 64K vertices while keeping 16-bit indices.
+// - 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 back-ends will typically ignore the VtxOffset/IdxOffset fields.
+// - The ClipRect/TextureId/VtxOffset fields must be contiguous as we memcmp() them together (this is asserted for).
 struct ImDrawCmd
 {
-    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[].
     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. Pre-1.71 or without ImGuiBackendFlags_RendererHasVtxOffset: always 0. With ImGuiBackendFlags_RendererHasVtxOffset: may be >0 to support meshes larger than 64K vertices with 16-bit indices.
+    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    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.
 
-    ImDrawCmd() { ElemCount = 0; TextureId = (ImTextureID)NULL; VtxOffset = IdxOffset = 0;  UserCallback = NULL; UserCallbackData = NULL; }
+    ImDrawCmd() { memset(this, 0, sizeof(*this)); } // Also ensure our padding fields are zeroed
 };
 
 // Vertex index, default to 16-bit
@@ -1977,17 +1979,17 @@ struct ImDrawList
     // [Internal, used while building lists]
     const ImDrawListSharedData* _Data;          // Pointer to shared draw data (you can use ImGui::GetDrawListSharedData() to get the one from current ImGui context)
     const char*             _OwnerName;         // Pointer to owner window's name for debugging
-    unsigned int            _VtxCurrentOffset;  // [Internal] Always 0 unless 'Flags & ImDrawListFlags_AllowVtxOffset'.
     unsigned int            _VtxCurrentIdx;     // [Internal] Generally == VtxBuffer.Size unless we are past 64K vertices, in which case this gets reset to 0.
     ImDrawVert*             _VtxWritePtr;       // [Internal] point within VtxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
     ImDrawIdx*              _IdxWritePtr;       // [Internal] point within IdxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
     ImVector<ImVec4>        _ClipRectStack;     // [Internal]
     ImVector<ImTextureID>   _TextureIdStack;    // [Internal]
     ImVector<ImVec2>        _Path;              // [Internal] current path building
+    ImDrawCmd               _CmdHeader;         // [Internal] Template of active commands. Fields should match those of CmdBuffer.back().
     ImDrawListSplitter      _Splitter;          // [Internal] for channels api (note: prefer using your own persistent instance of ImDrawListSplitter!)
 
     // If you want to create ImDrawList instances, pass them ImGui::GetDrawListSharedData() or create and use your own ImDrawListSharedData (so you can use ImDrawList without ImGui)
-    ImDrawList(const ImDrawListSharedData* shared_data) { _Data = shared_data; Flags = ImDrawListFlags_None; _VtxCurrentOffset = _VtxCurrentIdx = 0; _VtxWritePtr = NULL; _IdxWritePtr = NULL; _OwnerName = NULL; }
+    ImDrawList(const ImDrawListSharedData* shared_data) { _Data = shared_data; Flags = ImDrawListFlags_None; _VtxCurrentIdx = 0; _VtxWritePtr = NULL; _IdxWritePtr = NULL; _OwnerName = NULL; }
 
     ~ImDrawList() { ClearFreeMemory(); }
     IMGUI_API void  PushClipRect(ImVec2 clip_rect_min, ImVec2 clip_rect_max, bool intersect_with_current_clip_rect = false);  // Render-level scissoring. This is passed down to your render function but not used for CPU-side coarse clipping. Prefer using higher-level ImGui::PushClipRect() to affect logic (hit-testing and widget culling)