some details about dataset_util.ts

[Basasuya]

Hello author! I am a rookie in WebGL/OpenGL I have several problems through my reading in the code.

In https://github.com/tensorflow/tfjs-tsne/blob/master/src/dataset_util.ts#L36, I want to know why do you add 0.5 in line 36,37. Apart from this, why function dataTexCoordinates return vec2(col, row) not vec2(row, col)?

Thank you very much!