Add a test of light intensity vs. color

Models/PointLightIntensityTest/README.body.md

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+## Screenshot
+
+![screenshot](screenshot/screenshot-large.jpg)
+
+## Description
+
+This model tests various colors of point lights (via `KHR_lights_punctual`) that all share the same intensity (1.0).
+
+It is intended to serve as a test for the following clarification that was added to the `KHR_lights_punctual` extension:
+
+> The `intensity` represents the luminous intensity that the light would emit if it were colored pure white (`[1.0, 1.0, 1.0]`). The `color` property acts as a wavelength-specific multiplier.
+
+In other words, the specified color works as a "filter" in front of a light source of the specified intensity.
+
+The test material in each case is (0.8, 0.8, 0.8).  The lights are as follows:
+
+| Label | Color |
+|-------|-------|
+| Red                | (1.0, 0.0, 0.0) |
+| Green              | (0.0, 1.0, 0.0) |
+| Blue               | (0.0, 0.0, 1.0) |
+| Red + Green + Blue | Three colocated lights, matching the above |
+| White              | (1.0, 1.0, 1.0) |
+| Gray               | (0.5, 0.5, 0.5) |