Add renderer selection UI to all Gaussian splat examples

[mvaligursky]

Add a Renderer dropdown control to all unified Gaussian splat examples, allowing runtime selection between Auto, Raster (CPU Sort), Raster (GPU Sort), and Compute rendering modes.

Changes:

• Add Renderer dropdown UI to 20 Gaussian splat examples (matching the existing lod-streaming pattern)
• Create new controls files for 4 examples that previously had none (flipbook, multi-view, procedural-mesh, procedural-instanced)
• Fix renderer fallback UI feedback: when an unsupported renderer is selected (e.g. Compute on WebGL), the dropdown now correctly snaps back to the actual active renderer using deferred observer update
• Remove WebGPU fallback warning from GSplatParams renderer setter since the UI now handles the feedback visually

Examples updated:

• All 20 unified Gaussian splat examples: crop, editor, flipbook, global-sorting, lod-instances, lod-streaming, lod-streaming-sh, multi-view, paint, picking, procedural-instanced, procedural-mesh, procedural-shapes, reveal, shader-effects, shadows, viewer, weather, world, xr-views