The scene is drawn into a RenderTarget with
depth: 'texture'. Besides the colour, the GPU's depth
buffer is now a texture a shader can read. A second pass shows both,
colour on the left of the split and depth on the right; move the
pointer to move the split. The depth buffer does not store distance in
a straight line, so raw depth looks almost white everywhere;
linearized undoes the perspective projection. The white
background is the far plane.