[meta] Portable Ray Tracing

[Dzmitry Malyshau]

Stage I: #7

• Vulkan wrapper around acceleration structures
• Fix naga SPIRV-1.4 target - gfx-rs/naga#2230
• Add naga support for ray queries - gfx-rs/naga#2256
• Example

Stage II:

• metal-rs expose Metal-2 APIs to RT - gfx-rs/metal-rs#256
• naga MSL support for ray queries
• Metal support for acceleration structures and RT pipelines
• Make RT examples portable

Stage III:

• Support for RT pipelines in Vulkan and Metal-3
• Add naga support for RT shader stages
• Example