Bc nine respect smoothing groups (tinyobjloader#325)

* New vertex normal generation functions in viewer.cc that handle multiple smoothing groups correctly.

* Fix comiple on gcc.

* Update stb_image.h

* Add extra comment.

* Ubuntu 16.04 seems not availabe on Azure Pipeline anymore, so use `ubuntu-latest`.

Co-authored-by: Brian Collins <bc9@bcnine.com>

Author: syoyo

azure-pipelines.yml

@@ -28,8 +28,10 @@ jobs:
           black --check python/
         displayName: Check Python code format
 
+  # Ubuntu16.04 seems now deprecated(as of 2021/12/01),
+  # so use `ubuntu-latest`
   - job: linux
-    pool: {vmImage: "Ubuntu-16.04"}
+    pool: {vmImage: "ubuntu-latest"}
     steps:
       - task: UsePythonVersion@0
       - bash: |