This update makes wgpu a vendored dependency instead of having it in gfx/wgpu.

## Notes


It relies on https://phabricator.services.mozilla.com/D123157

It has a quirk related to OpenGL ES backend. Previousy, we manually had to disable GL backend
in order to avoid vendoring WASM dependencies in. This time, manual editing is more complicated,
so instead this change adds a few cargo patch lines to point WASM dependencies to dummy projects.

The update also totally removes SPIRV-Cross, since the latest `wgpu` doesn't depend on it any more.
The compiled binary size for Gecko should improve with this.

Differential Revision: https://phabricator.services.mozilla.com/D123153