Auto merge of #17891 - MortimerGoro:webgl_move, r=glennw,emilio

Improved WebGL architecture

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Info about the big picture and the goals of the WebGL refactor in this thread: https://groups.google.com/forum/#!topic/mozilla.dev.servo/0WMGz60kKzQ

I tried to reduce this PR as much as possible as requested in the thread. I'll do separate PRs for other features (e.g.: Batch messages or use shared memory to improve frame times) or fixes.

Some tips to ease the review process:
- Most changes in DOM objects follow the same pattern (remove CanvasMsg wrapper and use the new sender method).
- WebGLCommands are the same ones as before (moved from webrender_api). So those lines are already reviewed.
- See WebGL traits in [components/canvas_traits/webgl.rs](https://github.com/servo/servo/pull/17891/files#diff-8701045d01505418701d0631d4d45562)
- See WebGLThread and WR External Image bridge in [components/canvas/webgl_thread.rs](https://github.com/servo/servo/pull/17891/files#diff-281554879f39a2a041f7a69d442a5d2e)
- The implementation submitted in this PR creates a single `WebGLThread` for all ScriptThread/Pipelines. See that in [components/canvas/webgl_mode/inprocess.rs](https://github.com/servo/servo/pull/17891/files#diff-250070c6c5a38c7f9fa0f5b3c101f68b)

The conformance tests will help to guarantee that we don't miss anything.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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1 files changed, 6 insertions, 4 deletions

diff --git a/components/canvas/lib.rs b/components/canvas/lib.rs
index a3d90909637..b9f0823a07d 100644
--- a/components/canvas/lib.rs
+++ b/components/canvas/lib.rs
@@ -6,16 +6,18 @@
 
 extern crate azure;
 extern crate canvas_traits;
+extern crate compositing;
 extern crate cssparser;
 extern crate euclid;
 extern crate gleam;
 extern crate ipc_channel;
-#[macro_use]
-extern crate log;
+#[macro_use] extern crate log;
 extern crate num_traits;
 extern crate offscreen_gl_context;
-extern crate servo_config;
+extern crate webrender;
 extern crate webrender_api;
 
 pub mod canvas_paint_thread;
-pub mod webgl_paint_thread;
+pub mod gl_context;
+mod webgl_mode;
+pub mod webgl_thread;