WebGL gets a depth test, so 3D stops drawing in the wrong order

The software WebGL engine drew triangles in the order the page submitted them, with no depth buffer - so whenever a nearer surface was drawn before a farther one, the farther one painted right over it, and any real 3D scene came out with its layering scrambled. This release adds a real per-fragment depth test: every pixel now carries a depth, and an incoming fragment is kept or discarded by comparing its depth against what is already there, so the nearest surface wins no matter what order the page draws in - which is what makes solid 3D look solid. The full control set is here: enabling the depth test, clearing the depth buffer, choosing the comparison (the usual less-than and its seven siblings), and turning depth writes off for a transparent pass. With the depth test off, rendering is byte-for-byte what it was before, so nothing regresses. Proven end to end: a near blue triangle drawn before a far red one stays blue with the depth test on and goes red without it - depth order beating draw order, on demand - across three new tests. Web APIs moves 59 to 60; the overall holds at 56 (still a software renderer; render-to-texture is next).