Feature/stencil testing

[DougLilliequist]

@gordonnl Made a new fork which has a cleaner commit history now.

Will eventually make an example in OGL. Worth pointing out that it works with both depth testing and depth texture creation when using render targets (as shown in the gif)

As for implementation, here is how one would write to the stencil buffer:

const program = new Program(this.gl, {
  vertex,
  fragment,
  depthWrite: false,
  depthTest: false,
 })

program.setStencilFunc(this.gl.ALWAYS, 1, 0xff);
program.setStencilOp(this.gl.KEEP, this.gl.KEEP, this.gl.REPLACE);

And here is how an exisiting mesh can test against the stencil buffer. What is neat is that you don't need to make any drastic changes to your exisiting shaders:

this.sphere = new SphereMesh(this.gl);
this.sphere.program.setStencilFunc(this.gl.EQUAL, 2, 0xff);
this.sphere.program.setStencilOp(this.gl.KEEP, this.gl.KEEP, this.gl.KEEP);

[monolithMktg]

@DougLilliequist thanks for your amazing contributions :)