Add a test for alpha masking on vertex colors

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Install glTF Validator
       run: |
@@ -21,7 +21,7 @@ jobs:
         ./gltf_validator -r -a ./Models/
 
     - name: Upload Validation Reports
-      uses: actions/upload-artifact@v2
+      uses: actions/upload-artifact@v4
       with:
         name: validation-reports
         path: |

Models/VertexColorAlphaClipTest/README.body.md

@@ -0,0 +1,84 @@
+## Screenshot
+
+![screenshot](screenshot/screenshot-large.jpg)
+
+## Description
+
+This model tests what happens when vertex colors contain alpha values and a material uses `alphaCutoff`.  The above screenshot shows what this model looks like when the test passes.
+
+## Common Problems
+
+### Problem: Noisy alpha clipping
+
+![screenshot](screenshot/fail_alphaClipNoise.jpg)
+
+The two middle cubes (yellow and green) show a lot of fragment noise in the above screenshot. Here's a zoomed-in view to show detail on the green cube:
+
+![screenshot](screenshot/fail_noise_detail.png)
+
+The exact reason this noise is present can vary by rendering engine implementation, platform, and GPU in use.  It's possible that discarding masked fragments too early in a shader (prior to gradients and texture lookup usage) may be a contributing factor in some implementations.
+
+### Problem: No alpha mask
+
+![screenshot](screenshot/fail_noAlphaCutoff.jpg)
+
+The above screenshot shows a renderer that is not doing any alpha masking against vertex color alpha values.  In this case, a large red "X" appears over the left-most sample that is intended to be left blank.  This indicates a test failure.
+
+### Problem: Incorrect alpha comparison
+
+![screenshot](screenshot/fail_noEquality.jpg)
+
+In the above screenshot, the two middle cubes are missing, indicating that when the alpha value exactly matches the `alphaCutoff`, the result is clipped.
+
+The glTF specification [Alpha Coverage section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#alpha-coverage) has this to say:
+
+> When `alphaMode` is set to `MASK` the `alphaCutoff` property specifies the cutoff threshold. If the alpha value is greater than or equal to the `alphaCutoff` value then it is rendered as fully opaque, otherwise, it is rendered as fully transparent.
+
+The specification makes special mention of the equality case presented here by the two middle cubes in yellow and green.  They should be rendered fully opaque.  So a rendering similar to the above screenshot with the missing middle cubes is a test failure.
+
+## Label Text
+
+In case the text on the labels is hard to read or needs translation, it is reproduced below:
+
+### Far left sample
+
+```
+THIS SPACE INTENTIONALLY LEFT BLANK
+
+There should not be an X here.
+```
+
+```
+   Blank space
+  COLOR_0.a = 0.48
+alphaCutoff = 0.5
+   (less than)
+```
+
+### Second sample, yellow box
+
+```
+   Solid Box
+  COLOR_0.a = 0.5
+alphaCutoff = 0.5
+   (equal)
+```
+
+### Third sample, green box
+
+```
+   Solid Box
+  COLOR_0.a = 1.0
+alphaCutoff = 1.0
+   (equal)
+```
+
+### Far right sample, blue box
+
+```
+   Solid Box
+  COLOR_0.a = 0.51
+alphaCutoff = 0.5
+  (greater than)
+```
+