Support dual source blending

[freqmod]

Support writing shaders with alt blender mode.
See https://github.com/freqmod/helicoid/tree/master/helicoid-wgpu for usage example.

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file. TBD: This is a PR to get initial feedback.

Connections
Resolves #1804.
Depends on gfx-rs/naga#2427.

Description
See issue 1804 (linked above)

Testing
Tested on linux with OpenGL, and Vulkan, and on Mac using metal. I have made some changes for Direct X but since I don't have a windows build environment I haven't tested that.
Tested in the helicoid-wgpu crate in external repo, I guess a test or example could be made if we deem it useful.

[teoxoy]

Thanks for the PR!

[freqmod]

Thanks for the feedback, they make sense. I think i would like to finish the naga review first, when that is done i will look into these comments.

[freqmod]

I hope I have addressed the issues you pointed out.

I tested briefly with my simple application. It works when everything is set up correctly. It stops when the feature is not defined, and it complains when blend is enabled and the shader does not have an output with it.

I also ran the unittests and they seem to pass except one which also fails in trunk.

Cargo.toml conflicts as i have updated the hash to contain the naga changes.

[teoxoy]

Thanks for making the changes!
Besides a few naming/phrasing suggestions, I think the validation can be improved a bit more.

[teoxoy]

This should be the last round of review.
Note that the PR needs to be rebased.

[freqmod]

I have addressed the issues in this round and performed a rebase. If the assumptions in the comment to resource.rs lines 2924 to 2931 l are correct i hope indeed it is the last chages, if not i guess we may need some more rounds. All unittests except wgpu-test::wgpu-tests shader::struct_layout::uniform_input pass on linux, but that test also fails on trunk.

[freqmod]

seems like i have forgot clippy. Will fix, Fixed

[teoxoy]

I pushed a few more tweaks. Let me know if they look good and we can merge this.

Thank you for implementing this new functionality in naga & wgpu!

[freqmod]

The changes looks good to me, thanks for you patience with the multiple review rounds.