Fix WebGPU buffer memory leaks

[davepagurek]

Resolves #8341

Changes:

• Fix copy-and-paste error where the code was referring to this.renderer when this is already a renderer
• Makes sure that retained geometry reuses buffers instead of always allocating new ones
• Makes sure that immediate mode geometry all mark their buffers for reuse at the end of each frame

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated