Docking: Made DockBuilderSplitNode/DockNodeTreeSplit work even if the node doesn't have a size yet. (#2357, #2109)

Followup to fa0ce4b7d, at that time I came to the conclusion that programmatic split couldn't work without knowing the size ahead of it. I forgot the reason for that.  May bite us back!

imgui.cpp

@@ -11959,6 +11959,7 @@ static void ImGui::DockNodePreviewDockRender(ImGuiWindow* host_window, ImGuiDock
 
 void ImGui::DockNodeTreeSplit(ImGuiContext* ctx, ImGuiDockNode* parent_node, ImGuiAxis split_axis, int split_inheritor_child_idx, float split_ratio, ImGuiDockNode* new_node)
 {
+    ImGuiContext& g = *GImGui;
     IM_ASSERT(split_axis != ImGuiAxis_None);
 
     ImGuiDockNode* child_0 = (new_node && split_inheritor_child_idx != 0) ? new_node : DockContextAddNode(ctx, 0);
@@ -11976,6 +11977,7 @@ void ImGui::DockNodeTreeSplit(ImGuiContext* ctx, ImGuiDockNode* parent_node, ImG
     parent_node->VisibleWindow = NULL;
 
     float size_avail = (parent_node->Size[split_axis] - IMGUI_DOCK_SPLITTER_SIZE);
+    size_avail = ImMax(size_avail, g.Style.WindowMinSize[split_axis] * 2.0f);
     IM_ASSERT(size_avail > 0.0f); // If you created a node manually with DockBuilderAddNode(), you need to also call DockBuilderSetNodeSize() before splitting.
     child_0->SizeRef = child_1->SizeRef = parent_node->Size;
     child_0->SizeRef[split_axis] = ImFloor(size_avail * split_ratio);