Switch to imgui docking branch, vendor it

src/main.cpp

@@ -1,6 +1,26 @@
 #include "platform/platform.h"
 #include "renderer/renderer.h"
 #include "imgui.h"
+#include "imgui_internal.h"
+
+static void build_default_layout(ImGuiID dockspace_id)
+{
+    ImGui::DockBuilderRemoveNode(dockspace_id);
+    ImGui::DockBuilderAddNode(dockspace_id, ImGuiDockNodeFlags_DockSpace);
+    ImGui::DockBuilderSetNodeSize(dockspace_id, ImGui::GetMainViewport()->Size);
+
+    ImGuiID center = dockspace_id;
+    ImGuiID left   = ImGui::DockBuilderSplitNode(center, ImGuiDir_Left,  0.15f, nullptr, &center);
+    ImGuiID right  = ImGui::DockBuilderSplitNode(center, ImGuiDir_Right, 0.20f, nullptr, &center);
+    ImGuiID bottom = ImGui::DockBuilderSplitNode(center, ImGuiDir_Down,  0.25f, nullptr, &center);
+
+    ImGui::DockBuilderDockWindow("Browser",    left);
+    ImGui::DockBuilderDockWindow("Main",       center);
+    ImGui::DockBuilderDockWindow("Properties", right);
+    ImGui::DockBuilderDockWindow("Log",        bottom);
+
+    ImGui::DockBuilderFinish(dockspace_id);
+}
 
 int main(int argc, char **argv)
 {
@@ -27,6 +47,7 @@ int main(int argc, char **argv)
 
     int32_t last_w = w, last_h = h;
     bool show_demo = true;
+    bool first_frame = true;
 
     while (platform_poll_events(window)) {
         platform_get_size(window, &w, &h);
@@ -39,6 +60,39 @@ int main(int argc, char **argv)
         if (!renderer_begin_frame(renderer))
             continue;
 
+        // Full-window dockspace
+        ImGuiID dockspace_id = ImGui::GetID("MainDockSpace");
+        ImGui::DockSpaceOverViewport(dockspace_id, ImGui::GetMainViewport());
+
+        if (first_frame) {
+            build_default_layout(dockspace_id);
+            first_frame = false;
+        }
+
+        // Left panel
+        ImGui::Begin("Browser");
+        ImGui::Text("Instruments");
+        ImGui::Separator();
+        ImGui::End();
+
+        // Main content
+        ImGui::Begin("Main");
+        ImGui::Text("Main content area");
+        ImGui::End();
+
+        // Right panel
+        ImGui::Begin("Properties");
+        ImGui::Text("Details");
+        ImGui::Separator();
+        ImGui::End();
+
+        // Bottom panel
+        ImGui::Begin("Log");
+        ImGui::Text("Output / Log");
+        ImGui::Separator();
+        ImGui::End();
+
+        // Demo window
         if (show_demo)
             ImGui::ShowDemoWindow(&show_demo);