Fix up rendering logic in sample programs.

Author: oledfish

src/ImGui.NET.SampleProgram.XNA/ImGuiRenderer.cs

@@ -339,6 +339,11 @@ private unsafe void RenderCommandLists(ImDrawDataPtr drawData)
                 {
                     ImDrawCmdPtr drawCmd = cmdList.CmdBuffer[cmdi];
 
+                    if (drawCmd.ElemCount == 0) 
+                    {
+                        continue;
+                    }
+
                     if (!_loadedTextures.ContainsKey(drawCmd.TextureId))
                     {
                         throw new InvalidOperationException($"Could not find a texture with id '{drawCmd.TextureId}', please check your bindings");
@@ -360,19 +365,18 @@ private unsafe void RenderCommandLists(ImDrawDataPtr drawData)
 #pragma warning disable CS0618 // // FNA does not expose an alternative method.
                         _graphicsDevice.DrawIndexedPrimitives(
                             primitiveType: PrimitiveType.TriangleList,
-                            baseVertex: vtxOffset,
+                            baseVertex: (int)drawCmd.VtxOffset + vtxOffset,
                             minVertexIndex: 0,
                             numVertices: cmdList.VtxBuffer.Size,
-                            startIndex: idxOffset,
+                            startIndex: (int)drawCmd.IdxOffset + idxOffset,
                             primitiveCount: (int)drawCmd.ElemCount / 3
                         );
 #pragma warning restore CS0618
                     }
-
-                    idxOffset += (int)drawCmd.ElemCount;
                 }
 
                 vtxOffset += cmdList.VtxBuffer.Size;
+                idxOffset += cmdList.IdxBuffer.Size;
             }
         }
 

src/ImGui.NET.SampleProgram/ImGuiController.cs

@@ -520,12 +520,11 @@ private void RenderImDrawData(ImDrawDataPtr draw_data, GraphicsDevice gd, Comman
                             (uint)(pcmd.ClipRect.Z - pcmd.ClipRect.X),
                             (uint)(pcmd.ClipRect.W - pcmd.ClipRect.Y));
 
-                        cl.DrawIndexed(pcmd.ElemCount, 1, (uint)idx_offset, vtx_offset, 0);
+                        cl.DrawIndexed(pcmd.ElemCount, 1, pcmd.IdxOffset + (uint)idx_offset, (int)pcmd.VtxOffset + vtx_offset, 0);
                     }
-
-                    idx_offset += (int)pcmd.ElemCount;
                 }
                 vtx_offset += cmd_list.VtxBuffer.Size;
+                idx_offset += cmd_list.IdxBuffer.Size;
             }
         }