Add test of animated base color

Models/AnimatedColorsCube/README.body.md

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+## Screenshot
+
+![screenshot](screenshot/screenshot-large.gif)
+
+## Description
+
+This model tests the [`KHR_animation_pointer`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_animation_pointer) extension's effect on the `baseColorFactor` of a material.  When animated, the top cube should rotate above other colored cubes, and change its own color to match the cubes below.
+
+This model was created in Blender 4.2.0 by placing keyframes for a material's default base color and using the "Push Down Action" button in the NLA Track Editor.  The glTF export settings were configured with "Animation Mode" set to "NLA Tracks" and with a checkmark placed on "Animation Pointer (experimental)".
+
+## Possible Problems
+
+- If the top cube moves but stays red the entire time, it may indicate that `KHR_animation_pointer` is not supported in your renderer.
+
+- If the color changes become out-of-sync with the physical motion of the cube, it may indicate that the various animation tracks are not being driven by the same clock.  The movement is defined for the `[0, 3]` range while colors are defined for the `[0, 2.5]` range. This checks that the last color keyframe remains active until the movement track is complete. A similar test exists in the [BoxAnimated](../BoxAnimated) sample without any animation pointers.