use fewer shaders

[elalish]

While debugging WebXR mode I noticed we were recompiling some shaders that shouldn't. A couple of small fixes:

1. The placement box is transparent and was double-sided, which was causing it to be rendered twice (with two different shaders).
2. The shadow is turned off in AR until the model is set on the floor. Turning off the shadow recompiles the shader, so instead I turn it to a small (invisible) value.