Support Configurable Composite Alpha Modes, Transparency, & Renderer Options

[nicoburns]

Re-open of #70

Summary

This pull request introduces the ability to configure composite alpha modes and base colors across various renderers. Previously, the composite alpha mode was hardcoded (e.g. to Auto, which often defaults to Opaque), making alpha-blending and transparency difficult or impossible.

We now expose configurable option structs and allow specifying the background/clear colors and composite alpha modes for Vello, Vello Hybrid, Skia, and Pixels (CPU) renderers. Additionally, we make key options structs future-proof by marking them as #[non_exhaustive].

Note

softbuffer does not support transparency natively (yet), so these alpha-compositing features will apply only to the GPU and Pixels-based paths.

A small adjustment will need to be made in dioxus-native to use struct updates or Default::default() when constructing renderer options to accommodate the #[non_exhaustive] attribute.

Key Changes

1. Transparency & Alpha Compositing Control

• Vello Renderers: Added composite_alpha_mode option to vello's window_renderer.rs and vello-hybrid's window_renderer.rs. This controls the surface compositing behavior of the WGPU surface inside the respective window renderers.
• Pixels CPU / Raster Renderers: Added configuration options via the PixelsRendererOptions struct inside lib.rs to customize the base_color and composite_alpha_mode using the PixelsBuilder API.
• Skia Renderer: Added configuration options via the SkiaRendererOptions inside window_renderer.rs to customize the base_color only as Skia handles transparency by default.
• Skia Raster Renderer: Clears the canvas with Color::TRANSPARENT in image_renderer.rs by default to preserve alpha channels in rendered outputs.

2. Extensibility & Future Proofing

• Marked options in vello's window_renderer.rs and vello-hybrid's window_renderer.rs as #[non_exhaustive] to allow adding more configuration fields in the future without breaking user builds.

3. Crate-specific Options & Structs

• anyrender_vello_cpu: Exported VelloCpuRendererOptions as an alias for PixelsRendererOptions.
• anyrender_skia: Exported SkiaRasterRendererOptions as an alias for PixelsRendererOptions.

Breaking Changes

• The configuration structs VelloRendererOptions and VelloHybridRendererOptions are now #[non_exhaustive]. Constructing these structs directly without using Default::default() or struct update syntax will cause compilation errors.

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