Refactor RgbVis with shader

[axelboc]

Driven by the deprecation of RGBFormat #962 and the optimisation of the BGR flip.

An RGB image is represented as an array of size [rows, cols, 3], where the three values in the last dimension are for the red, green and blue channels (or blue, green and red). There's no alpha values, so if we want to use the typed array returned by the back-end provider as is, without looping through it (for performance reasons), we can't just switch to RGBAFormat.

So instead, we use a 3D texture with REDFormat and access each color component in a custom fragment shader with the texture() function. Three's DataTexture3D is still under development, so we have to set the format and type manually: https://github.com/mrdoob/three.js/blob/master/src/textures/DataTexture3D.js

The cast/conversion to a compatible texture type remains the same as before.

The BGR flip is now done in the shader -- we no longer call createArrayFromView.