use load-model event for 3d tiles blending options

[kfarr]

Blending mode changes exhibit "popping" and issues sometimes where newly loaded tiles don't have blending applied or there is a delay before it is applied. This could probably be optimized to more closely follow the custom material demo here

tiles.addEventListener( 'load-model', onLoadModel );

https://github.com/NASA-AMMOS/3DTilesRendererJS?tab=readme-ov-file#custom-material