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, 7 insertions, 0 deletions

diff --git a/source/core/smart-pointer.h b/source/core/smart-pointer.h
index 4c6744d1b..f45618050 100644
--- a/source/core/smart-pointer.h
+++ b/source/core/smart-pointer.h
@@ -199,6 +199,13 @@ namespace Slang
             return pointer;
         }
 
+        void attach(T* p)
+        {
+            T* old = pointer;
+            pointer = p;
+            releaseReference(old);
+        }
+
         T* detach()
         {
             auto rs = pointer;