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, 28 insertions, 0 deletions

diff --git a/tools/gfx/gui.h b/tools/gfx/gui.h
new file mode 100644
index 000000000..cbec5796f
--- /dev/null
+++ b/tools/gfx/gui.h
@@ -0,0 +1,28 @@
+// gui.h
+#pragma once
+
+#include "render.h"
+#include "vector-math.h"
+#include "window.h"
+
+#include "external/imgui/imgui.h"
+
+namespace gfx {
+
+struct GUI : RefObject
+{
+    GUI(Window* window, Renderer* renderer);
+    ~GUI();
+
+    void beginFrame();
+    void endFrame();
+
+private:
+    RefPtr<Renderer>            renderer;
+    RefPtr<PipelineState>       pipelineState;
+    RefPtr<DescriptorSetLayout> descriptorSetLayout;
+    RefPtr<PipelineLayout>      pipelineLayout;
+    RefPtr<SamplerState>        samplerState;
+};
+
+} // gfx