[RFC] Roadmap to a smaller, faster, and cleaner Unified Device API

[bobcao3]

The long term goal for the Unified Device API, or the RHI, is to be isolated into its own repo and maintained separately from Taichi. Where we will create its own unit tests and integration tests to improve robustness and maintain strong isolation boundary between them.

In the end, we want to achieve these goals for the future RHI:

1. RHI can be trimmed down through discarding unused modules. We should make RHI per-backend dynamically linked and optionally statically linked.
2. RHI contains its own full functional specification and tests that uphold these specifications
3. Provide a zero-overhead C-API
4. Do not leak symbols to linked libraries & support different drivers and external dependencies robustly
5. Able to use shared library if the client of RHI already uses them
6. Being a fully functioned RHI that we can build other non-taichi applications on top of it
7. Make it smaller in size and complexity

To achieve this goal while not hugely breaking the workflow and goals within Taichi, we will start by progressively trimming away dependencies of RHI. This hopefully will eventually lead to a clean separation when we move RHI outside of Taichi repo. Here are the first steps:

• [ ] Remove all Taichi IR and related dependencies (ir.h, program.h, LLVM, eigen, etc.)
  • [x] GFX
  • [ ] LLVM
• [ ] Remove / avoid TI_XXX and spdlog based logging when required (use MACRO, log less, and use what's provided by standard library)
  • [ ] GFX
  • [ ] LLVM
• [ ] Remove exceptions (use noexcept) and use clear status return values. (This reduces generated code size and helps with C-API impl)
  • [ ] GFX
  • [ ] LLVM
• [ ] Re-organize the API to make most-of-the-time optional features being binary-wise optional. (e.g. Device/Context creation, Window/Surface creation, etc.)
• [ ] Re-organize the binding model to help with integration into 3rd party code bases

[bobcao3]

Here to track the progress on the API surface (adding return status, adding documentation & specs, removing STL, or other changes):

• [x] ShaderResourceSet
  • [x] Split into shader resource set and raster-state set
  • [x] rw_buffer
  • [x] buffer
  • [x] image
  • [x] rw_image
• [x] RasterResources
  • [x] vertex_buffer
  • [x] index_buffer
• [ ] Pipeline
• [ ] CommandList
  • [x] bind_pipeline
  • [x] bind_resources
  • [x] buffer_barrier
  • [x] memory_barrier
  • [x] buffer_copy
  • [x] buffer_fill
  • [ ] dispatch
  • [ ] begin_renderpass
  • [ ] end_renderpass
  • [ ] draw
  • [ ] draw_instance
  • [ ] set_line_width
  • [ ] draw_indexed
  • [ ] draw_indexed_instance
  • [ ] image_transition
  • [ ] buffer_to_image
  • [ ] image_to_buffer
  • [ ] copy_image
  • [ ] blit_image
  • [ ] signal_event
  • [ ] reset_event
  • [ ] wait_event
• [ ] Stream
  • [x] new_command_list
  • [ ] submit
  • [ ] submit_synced
  • [ ] command_sync
  • [ ] device_time_elapsed_us Redesign?
• [ ] Device
  • [ ] allocate_memory
  • [ ] dealloc_memory
  • [ ] get_memory_physical_pointer
  • [ ] create_pipeline
  • [ ] create_event
  • [ ] allocate_memory_unique
  • [ ] fetch_result_uint64 Remove
  • [ ] get_compute_stream
  • [ ] wait_idle
  • [x] map_range
  • [x] map
  • [x] unmap
  • [ ] share_to Redesign?
  • [ ] memcpy_internal Move / Redesign?
  • [ ] check_memcpy_capability Move / Redesign?
  • [ ] memcpy_direct Move / Redesign?
  • [ ] memcpy_via_staging Move / Redesign?
  • [ ] memcpy_via_host Move / Redesign?
  • [ ] arch
  • [ ] get_caps
  • [ ] set_caps
• [ ] Surface
  • [ ] acquire_next_image
  • [ ] get_target_image
  • [ ] present_image
  • [ ] get_size
  • [ ] get_image_count
  • [ ] image_format
  • [ ] resize
  • [ ] get_depth_data Move / Redesign
  • [ ] get_image_data Move / Redesign
• [ ] GraphicsDevice (Merge with Device?)
  • [ ] create_raster_pipeline
  • [ ] get_graphics_stream
  • [ ] create_surface Redesign?
  • [ ] create_image
  • [ ] destroy_image
  • [ ] image_transition Remove?
  • [ ] buffer_to_image Move / Redesign
  • [ ] image_to_buffer Move / Redesign

[bobcao3]

DX11 Impl tracker: #6879 Vulkan Impl tracker: #6880 OpenGL Impl tracker: #6881 Metal Impl tracker: #6882