Node: escape hatch for more advanced WebGL stuff

Node should expose a more generic escape hatch to use gl and do any WebGL stuff.

Node should expose a basic way to implement your own `draw()`. that way, people can do whatever they want with `gl`, but the contract should be clear (should you still receive the uniforms? is the shader in the scope? should there be a more generic contract that Node implements?). The simplest example is to call `gl.clear()` to fill a color. Try regl in this paradigm. Try Three.js in this paradigm.

The main problem is most WebGL libs are intrusive and alter the gl state (e.g. they don't expect to just draw in the current bounded framebuffer).

e.g. https://github.com/mrdoob/three.js/issues/7483#issuecomment-264195243