Bug 1575008 - WebGPU implementation basis r=webidl,baku

This change vendors `wgpu` library in-tree and hooks up the initialization bits. It implements adapter and device initialization and adds a simple test.

Current status:
  - [x] Architecture
    - [x] figure out the IPC story
    - [ ] move wgpu crates into a dedicated folder (let's follow up with this)
  - [x] Review
    - [x] WebIDL changes by DOM peers
  - [x] Linux
    - [x] avoid depending on spirv_cross - gfx-rs/wgpu#371
  - [x] macOS
    - [x] due to cross-compiling shaders - gfx-rs/gfx#3047
    - [x] need the dependency update
    - [x] stop using gcc - SSheldon/rust-objc-exception#5
    - [x] unexpected SSL header collision - https://phabricator.services.mozilla.com/D51148
    - [x] undefined Metal symbols
    - [x] missing webrtc headers for IPDL magic - https://phabricator.services.mozilla.com/D51558
  - [x] Windows
    - [x] due to "ipc-channel" not supporting Windows yet - servo/ipc-channel#233
    - [x] due to some exceptional stuff - grovesNL/spirv_cross#121
    - [x] undefined symbol: `D3D12CreateDevice`
    - [x] d3d12.dll is not found, dxgi1_4 doesn't present
    - [x] d3d11.dll and dxgi.dll need to be explicitly loaded on win32 mingw - gfx-rs/gfx#3076
    - [x] libbacktrace fails to link on win32 mingw
    - [x] cc mislinking C++ standard library - rust-lang/cc-rs#455
  - [x] Android
    - [x] spirv-cross fails to build - KhronosGroup/SPIRV-Cross#1193

Update-1:
We decided to go with IPDL mechanism instead of Rust based ipc-channel (or any alternatives), which unblocks Windows build.

Update-2:
It appears that WebGPUThreading isn't needed any more as the child thread (and its event loop) is now managed by IPDL infrastructure. This PR removes it 🎉 .

Update-3:
InstanceProvider is also removed.

Update-4:
All set, the try is green, waiting for dependent changes to go in.

Differential Revision: https://phabricator.services.mozilla.com/D49458

UltraBlame original commit: cbec8a55a1bbad484ea0628b527de6f0cc7dc5f7

Author: marco-c

.cargo/config.in

@@ -7,6 +7,16 @@ branch = "master"
 git = "https://github.com/mozilla/neqo"
 replace-with = "vendored-sources"
 
+[source."https://github.com/kvark/spirv_cross"]
+branch = "wgpu-test"
+git = "https://github.com/kvark/spirv_cross"
+replace-with = "vendored-sources"
+
+[source."https://github.com/kvark/rust-objc-exception"]
+branch = "cc"
+git = "https://github.com/kvark/rust-objc-exception"
+replace-with = "vendored-sources"
+
 [source."https://github.com/hsivonen/packed_simd"]
 branch = "rust_1_32"
 git = "https://github.com/hsivonen/packed_simd"

.clang-format-ignore

@@ -38,6 +38,7 @@ layout/style/nsStyleStructList.h
 # Autogenerated file
 gfx/gl/GLConsts.h
 gfx/webrender_bindings/webrender_ffi_generated.h
+dom/webgpu/ffi/wgpu_ffi_generated.h
 intl/unicharutil/util/nsSpecialCasingData.cpp
 intl/unicharutil/util/nsUnicodePropertyData.cpp
 intl/unicharutil/util/nsUnicodeScriptCodes.h