Window transparency support

[kvark]

There is a bit of a portability problem: Vulkan/Mesa supports VK_COMPOSITE_ALPHA_PRE_MULTIPLIED_BIT_KHR, while Metal supports VK_COMPOSITE_ALPHA_POST_MULTIPLIED_BIT_KHR.

We don't want to do an extra full-screen pass, and we don't control the pipelines of the user.
So perhaps we could request transparency and then get one of the modes back:

#[derive(Debug)]
pub struct SurfaceConfig {
    ...
    transparent: bool,
}


enum AlphaMode {
  Ignored,
  PreMultiplied,
  PostMultiplied,
}

struct SurfaceInfo {
  format: TextureFormat,
  alpha: AlphaMode,
}

fn resize(&self, config: SurfaceConfig) -> SurfaceInfo {...}

Then the user of the library can do the pre (or post) multiplication themselves in the shader.

[jansol]

Vulkan also has the concept of "passthrough" alpha, which basically signals that it will be taken care of out of band. I don't remember any implementations that actually report that as supported though.

[kvark]

heads up - I'm working on this