Refactor Aconite's shader-handling code

[colinbdclark]

At the moment, it's pretty awkward to work with Aconite shaders. They are somewhat hidden behind the aconite.glRenderer interface, which requires odd additional options for uniforms in addition to a model, as well as layers of wrapping of arrays that is confusing and error prone. Some useful comments about the issues can be found in Aconite's source code as well as the video example that was created for an older version of our Onward paper.

We should introduce a somewhat closer to the metal component that represents a GL shader and that is fully modelized, and which is responsible for loading (via a separate component), compiling, and attaching shaders as well as managing their runtime state.