Add a canvas to show the output of the render-to-texture operation (read from the texture framebuffer before it gets unbound).  Set the output texture as the source for the main rendering loop.  Set the texture bounds as a power of two (*4, not *3), so that the render actually works, as per a warning from FF's webgl.  (Apparently a power-of-2 viewport is insufficient, and the actual texture needs to be that size.)  Make the Tree.texture member be non-underscored since it's now used outside the Tree code.