Incorrect dithering when running blend-clipped test

[dati91]

The filters/blend-clipped test uses linear gradient shader, which uses the dithering feature.
The dithering works for the gradient tests, but fails with this one.
If I hflip the dither matrix we get a result which pretty close to what we should see. (gl_wr 1026 vs vk_wr 1335 differences)
But messes up the other tests.

We should double check our transformations (and why we differ from the original webrender).