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, 26 insertions, 7 deletions

diff --git a/examples/model-viewer/cube.obj b/examples/model-viewer/cube.obj
index 7226aaa77..2f7de8a92 100644
--- a/examples/model-viewer/cube.obj
+++ b/examples/model-viewer/cube.obj
@@ -14,11 +14,30 @@ vn  1.000000  0.000000  0.000000
 vn  0.000000  0.000000  1.000000
 vn -1.000000  0.000000  0.000000
 vn  0.000000  0.000000 -1.000000
-usemtl Material
+
+v -10 -1 -10
+v  10 -1 -10
+v  10 -1  10
+v -10 -1  10
+vn 0 1 0
+
+usemtl Red
 s off
-f 1//1 2//1 3//1 4//1
-f 5//2 8//2 7//2 6//2
-f 1//3 5//3 6//3 2//3
-f 2//4 6//4 7//4 3//4
-f 3//5 7//5 8//5 4//5
-f 5//6 1//6 4//6 8//6
+f 2//3 6//3 5//3 1//3
+f 4//5 8//5 7//5 3//5
+
+usemtl Green
+s off
+f 4//1 3//1 2//1 1//1
+f 6//2 7//2 8//2 5//2
+
+usemtl Blue
+s off
+f 3//4 7//4 6//4 2//4
+f 8//6 4//6 1//6 5//6
+
+o Ground
+usemtl Ground
+s off
+f 9//7 10//7 11//7 12//7
+