texture-npot-video.html seems to produce invalid values for premultiplied alpha

[texture-npot-video.html](http://khronos.org/registry/webgl/sdk/tests/conformance/textures/misc/texture-npot-video.html) seems to produce invalid values for premultiplied alpha (color values exceed alpha values, e.g. (1, 1, 1, 0)).

I haven't looked at the test case itself, but it triggers a Chrome OS bug that seems to be caused by invalid color values: https://crbug.com/914204

Start with texture-npot-video.html and inspect how it checks premultiplied alpha values. Reproduce the reported values, including (1, 1, 1, 0), and compare them with the test's expected results and the linked Chrome OS bug; done means the test no longer produces invalid color values.

Written by the indexing model from the issue text.