Remove wgpu feature. Add wgpu_27 and wgpu_28 features.

[nicoburns]

Objective

Allow the same version of Vello to compile with multiple versions of WGPU (current v27 and v28):

• This should make it easier to upgrade to new versions of WGPU promptly without worrying about leaving users who may still need an older WGPU version behind.
• In many cases, it may also make upgrading WGPU a non-breaking change.

Prior Art

The i-slint-core crate has similar feature flags for WGPU versions (https://docs.rs/crate/i-slint-core/latest/features)

Changes made

• Replace the wgpu and naga feature flags in the vello, vello_hybrid, vello_shaders and vello_sparse_shaders crates with wgpu_27, wgpu_28 (and naga_27, naga_28) feature flags.
• Re-export e.g. wgpu_28 as wgpu in the crate roots
• The wgpu-profiler and hot-reload debug features are currently configured to only work with the latest WGPU version.

Todo

• Use crates.io version of wgpu-profiler once Update wgpu dependency to version 28 Wumpf/wgpu-profiler#103 lands

[waywardmonkeys]

I'd like to see this happen and get a release out!

Do you want to update this or should someone else take it up?

cc: @xStrom @PoignardAzur

[xStrom]

I'm not sure if it's worth actually supporting multiple wgpu versions. However, just as a technical note, the features should also work if both are selected. Probably the highest version should win then. So in this PR if both wgpu_27 and wgpu_28 are enabled, it should work the same way as if just wgpu_28 was enabled. This keeps --all-features functional.

[PoignardAzur]

I don't have a strong opinion on this, but it seems like a lot of churn.

[PoignardAzur]

Leftover comment?