ImDrawList: Simplified code and added check to avoid mis-recycling a callback command.

imgui_draw.cpp

@@ -176,8 +176,8 @@ void ImDrawList::UpdateClipRect()
     }
 
     // Try to merge with previous command if it matches, else use current command
-    ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? &CmdBuffer.Data[CmdBuffer.Size-2] : NULL;
-    if (prev_cmd && memcmp(&prev_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) == 0 && prev_cmd->TextureId == GetCurrentTextureId())
+    ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? curr_cmd - 1 : NULL;
+    if (prev_cmd && memcmp(&prev_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) == 0 && prev_cmd->TextureId == GetCurrentTextureId() && prev_cmd->UserCallback == NULL)
         CmdBuffer.pop_back();
     else
         curr_cmd->ClipRect = curr_clip_rect;
@@ -195,8 +195,8 @@ void ImDrawList::UpdateTextureID()
     }
  
     // Try to merge with previous command if it matches, else use current command
-    ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? &CmdBuffer.Data[CmdBuffer.Size-2] : NULL;
-    if (prev_cmd && prev_cmd->TextureId == curr_texture_id && memcmp(&prev_cmd->ClipRect, &GetCurrentClipRect(), sizeof(ImVec4)) == 0)
+    ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? curr_cmd - 1 : NULL;
+    if (prev_cmd && prev_cmd->TextureId == curr_texture_id && memcmp(&prev_cmd->ClipRect, &GetCurrentClipRect(), sizeof(ImVec4)) == 0 && prev_cmd->UserCallback == NULL)
         CmdBuffer.pop_back();
     else
         curr_cmd->TextureId = curr_texture_id;