Unified GSplat support for rendering to shadow map + Example

[mvaligursky]

• This PR adds support for unified rendering to shadow map. When objects a layer renders to both forward and shadow camera are the same, a single shared work-buffer is used both both, otherwise those are independent
• fixed ordering of shadow-catcher to use drawBucket
• renderer now provides this uniform automatically, removed manual setup by splat rendering:
  uniform vec4 viewport_size; // viewport width, height, 1/width, 1/height
• gsplat shader now correctly skips splat culling behind camera for otho mode
• adding a new simple material API to configure rendering of gsplats in work buffer. Before, an event was triggered for each instance of material to be set up. Now we have a single material in GSplatParams, and internally values are copied as needed.

• new example to demonstrate unified shadows + shadow catcher + projected skydome