Multiple backends

[lukaslihotzki]

This draft shows an approach how both Vulkan and GLES can be supported by a single executable. In the first commit, all traits specific to the backend are moved from the crate root to the common directory, which is included in both backends as separate modules using path attributes. In the following commits, the traits are made more abstract from specific backends:

1. The dependency of HasShaderBinding and ShaderBindable is inverted, so now, ShaderBindable depends on HasShaderBinding. Practically, none or both traits are implemented, so it makes no difference. But with this change, HasShaderBinding can be implemented independently of the backend.
2. The traits ResourceDevice and CommandDevice are made independent from the backend by making TextureViewDesc generic on the inner Texture.
3. Buffer handling becomes generic by introducing the Buffer trait. This allows to make the belt in blade-util generic over multiple backends.
4. The blade macro ShaderData is duplicated per backend and then re-exported in the blade-graphics crate, to make it easy to choose the matching one. This is gated by the derive feature.

These changes are sufficient to support multiple backends in Zed, for example (zed-industries/zed#14931). However, blade could be made more generic with extra traits (for example, Texture handling).

fixes #130

[kvark]

Hi @lukaslihotzki and thank you for experimenting with this!

The graphics library has a fairly large API. Making it polymorphic across backends is taking a tall at the code clarity and quality.

I wonder if we can expose it such that the user would enable "gles" as a feature and see this API:

blade_graphics::Device; // main device
blade_graphics::fallback::Device; // GLES device

So essentially GLES feature would bring another implementation exposed inside "fallback" namespace, and the user can use both. That would allow Zed (and other users) to potentially have different renderers, e.g. BladeRenderer and BladeFallbackRenderer, thus shifting the polymorphism to a higher level.

[lukaslihotzki]

With #150 and #151, the diff shrinked from +434 −352 to +321 −254, so some progress is made on this. Nice!

[theofficialgman]

Any interest in picking this back up? This is a blocker for zed on many devices (already mentioned in the issue reports related to this).