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, 1 insertions, 1 deletions

diff --git a/premake5.lua b/premake5.lua
index 1c900c876..73c7f65e2 100644
--- a/premake5.lua
+++ b/premake5.lua
@@ -394,7 +394,7 @@ tool "gfx"
     -- rather than a stand-alone executable.
     kind "StaticLib"
 
-    includedirs { ".", "external", "source" }
+    includedirs { ".", "external", "source", "external/imgui" }
 
     filter { "system:windows" }