[Feature] Default pipelines to opaque blending (blend: None)

[vmarcella]

Overview

Pipelines always enable ALPHA_BLENDING, which adds costs on some GPUs when blending isn't needed. Most geometry is opaque and doesn't require blending, so the default should be blend: None (replace) with an opt-in with_blend(state) method for cases that need transparency.

Current State

RenderPipelineBuilder::build() hardcodes ALPHA_BLENDING for all color targets:

// crates/lambda-rs-platform/src/wgpu/pipeline.rs
let color_targets: Vec<Option<wgpu::ColorTargetState>> =
  match &self.color_target_format {
    Some(fmt) => vec![Some(wgpu::ColorTargetState {
      format: *fmt,
      blend: Some(wgpu::BlendState::ALPHA_BLENDING),
      write_mask: wgpu::ColorWrites::ALL,
    })],
    None => Vec::new(),
  };

This forces alpha blending overhead even for fully opaque geometry, which can hurt performance on tile-based GPUs (mobile, Apple Silicon) where blending requires additional memory bandwidth.

Scope

Goals:

• Default to blend: None (opaque/replace mode) for better default performance
• Expose with_blend(BlendState) method on RenderPipelineBuilder for opt-in blending
• Maintain backwards compatibility for users who need alpha blending

Non-Goals:

• Automatic blend state detection based on shader analysis
• Per-attachment blend state configuration (future work)

Proposed API

// In crates/lambda-rs-platform/src/wgpu/pipeline.rs
pub enum BlendMode {
  /// No blending, replace destination (default).
  None,
  /// Standard alpha blending.
  AlphaBlending,
  /// Premultiplied alpha blending.
  PremultipliedAlpha,
  /// Additive blending.
  Additive,
  /// Custom blend state.
  Custom(wgpu::BlendState),
}

impl RenderPipelineBuilder<'a> {
  /// Set the blend mode for the color target. Defaults to `BlendMode::None`.
  pub fn with_blend(mut self, mode: BlendMode) -> Self {
    self.blend_mode = mode;
    return self;
  }
}

Example Usage:

let pipeline = RenderPipelineBuilder::new()
  .with_color_target(TextureFormat::Bgra8Unorm)
  .with_blend(BlendMode::AlphaBlending) // Opt-in for transparent geometry
  .build(&gpu, &vertex_shader, Some(&fragment_shader));

Acceptance Criteria

• RenderPipelineBuilder defaults to blend: None
• with_blend(BlendMode) method added to RenderPipelineBuilder
• BlendMode enum provides common presets (None, AlphaBlending, PremultipliedAlpha, Additive)
• BlendMode::Custom(wgpu::BlendState) variant for advanced use cases
• Existing examples updated to use with_blend() where transparency is needed
• Documentation updated with blending guidance
• Unit test verifying default blend state is None

Affected Crates

lambda-rs-platform

Notes

• This is a breaking change for users relying on implicit alpha blending; migration is straightforward (add .with_blend(BlendMode::AlphaBlending))
• Future work: per-attachment blend states for MRT (Multiple Render Targets)