Improve model-viewer support for lights (#626)

* Improve model-viewer support for lights

The main visible change here is that the model-viewer example supports
multiple light sources, with a basic UI for adding new light sources to
the scene, and for manipulating the ones that are there.

Along the way I also refactored the `IMaterial` decomposition to be a
bit less naive, while still only including a completely naive
Blinn-Phong implementation.

I also went ahead and spruced up the `cube.obj` file so that it has
multiple materials, although it is still a completely uninteresting
asset.

* Fixup: Windows SDK version

1 files changed, 1 insertions, 1 deletions

diff --git a/source/slang/check.cpp b/source/slang/check.cpp
index 3c7bcaf84..c5ebf7f30 100644
--- a/source/slang/check.cpp
+++ b/source/slang/check.cpp
@@ -888,7 +888,7 @@ namespace Slang
                         {
                             if (diagSink)
                             {
-                                diagSink->diagnose(typeExp.exp.Ptr(), Diagnostics::unimplemented, "can't fill in default for generic type parameter");
+                                diagSink->diagnose(typeExp.exp.Ptr(), Diagnostics::genericTypeNeedsArgs, typeExp);
                                 *outProperType = getSession()->getErrorType();
                             }
                             return false;