Internals: Initialize drawlist earlier in Begin() to facilitate detec…

…ting accidental draw earlier than legal. (#3311)

imgui.cpp

@@ -5606,6 +5606,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->HasCloseButton = (p_open != NULL);
         window->ClipRect = ImVec4(-FLT_MAX,-FLT_MAX,+FLT_MAX,+FLT_MAX);
         window->IDStack.resize(1);
+        window->DrawList->_ResetForNewFrame();
 
         // Restore buffer capacity when woken from a compacted state, to avoid
         if (window->MemoryCompacted)
@@ -5883,7 +5884,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // DRAWING
 
         // Setup draw list and outer clipping rectangle
-        window->DrawList->_ResetForNewFrame();
+        IM_ASSERT(window->DrawList->CmdBuffer.Size == 1 && window->DrawList->CmdBuffer[0].ElemCount == 0);
         window->DrawList->PushTextureID(g.Font->ContainerAtlas->TexID);
         PushClipRect(host_rect.Min, host_rect.Max, false);