Add basic support for "Dear IMGUI" (#625)

This isn't being made visible just yet, but it will allow us to have a simple UI for loading models into the model-viewer example.

In order to support rendering with IMGUI I had to add the following to the `Renderer` layer:

* viewports
* scissor rects
* blend support

These are really only fully implemented for D3D11, but adding them to the other back-ends should be a reasonably small task.

1 files changed, 33 insertions, 0 deletions

diff --git a/tools/gfx/window.cpp b/tools/gfx/window.cpp
index ee9f50813..9bc63b896 100644
--- a/tools/gfx/window.cpp
+++ b/tools/gfx/window.cpp
@@ -111,22 +111,53 @@ int runWindowsApplication(
 struct Window
 {
     HWND handle;
+    WNDPROC nativeHook;
 };
 
+void setNativeWindowHook(Window* window, WNDPROC proc)
+{
+    window->nativeHook = proc;
+}
+
 static LRESULT CALLBACK windowProc(
     HWND    windowHandle,
     UINT    message,
     WPARAM  wParam,
     LPARAM  lParam)
 {
+    Window* window = (Window*) GetWindowLongPtrW(windowHandle, GWLP_USERDATA);
+
+    // Give the installed filter a chance to intercept messages.
+    // (This is used for ImGui)
+    if( window )
+    {
+        if(auto nativeHook = window->nativeHook)
+        {
+            auto result = nativeHook(windowHandle, message, wParam, lParam);
+            if(result)
+                return result;
+        }
+    }
+
     // TODO: Actually implement some reasonable logic here.
     switch (message)
     {
+    case WM_CREATE:
+        {
+            auto createInfo = (CREATESTRUCTW*) lParam;
+            window = (Window*) createInfo->lpCreateParams;
+            window->handle = windowHandle;
+
+            SetWindowLongPtrW(windowHandle, GWLP_USERDATA, (LONG)window);
+        }
+        break;
+
     case WM_CLOSE:
         PostQuitMessage(0);
         return 0;
     }
 
+
     return DefWindowProcW(windowHandle, message, wParam, lParam);
 }
 
@@ -159,6 +190,8 @@ static ATOM getWindowClassAtom()
 Window* createWindow(WindowDesc const& desc)
 {
     Window* window = new Window();
+    window->handle = nullptr;
+    window->nativeHook = nullptr;
 
     OSString windowTitle(desc.title);