vello_gpu: Add initial support for excluding certain paints from shaders at compile-time

[LaurenzV]

As our experiments have shown, complex shaders have a bad impact on performance as well as shader compilationt times on low-tier devices. This PR proposes a new method for allowing users to exclude certain features at compile-time, if they know they won't need it. In case they do still use it, we will panic at runtime.

This PR is best reviewed commit-by-commit.

• The first commit changes the structure of the shaders by extracting some code into helper methods, making it easier to later on compile out certain parts of the render.wesl module.
• The second commit extracts some functionality of vello_tests into a custom crate, so that it can be reused by different tests. This is necessary because we want to be able to test the various configurations of enabled feature sets, which (as you will see) is easier to do by creating a new test crate, instead of modifying vello_tests. Extracting this part of the code allows us to reuse the snapshot logic between the two.
• The third commit actually implements the main logic of this PR. For now, I've only implemented this for blurred rects, sweep gradients and bicubic image rendering, which is what we personally don't need. In theory, this can of course be extended to cover other paints, such as linear gradients. However, if we for example exclude all gradients, we would also have to start feature gating stuff on the Rust side, as some of the bindings might become dead code and would have to be excluded as well. I wanted to avoid this for the first prototype, so I've only limited the implementation to these three paints for now.
• The fourth commit adds a new test crate and a CI step that ensures we don't accidentally break anything for certain feature configurations. Let me know if you think they are unnecessary and I should remove them again.

[LaurenzV]

Bit annoying having to redefine this everywhere. 😅 Not sure if we should figure out something else.

[LaurenzV]

Running this on Ubuntu on purpose instead of MacOS, as MacOS runs the whole workspace with all-feature, and that requires unnecessarily recompiling a lot of dependencies because the enabled features change.