[Impeller] framebuffer_blend.frag checks supports_decal > 1 instead of > 0.0, always taking the software decal fallback #196266

Description

In engine/src/flutter/impeller/entity/shaders/blending/framebuffer_blend.frag, the supports_decal specialization constant (layout(constant_id = 1) const float supports_decal = 1;) is specialized in ContentContext to 0.0 or 1.0 based
on Capabilities::SupportsDecalSamplerAddressMode().

However, Sample() in framebuffer_blend.frag checks if (supports_decal > 1) instead of if (supports_decal > 0.0):

vec4 Sample(sampler2D texture_sampler, vec2 texture_coords) {
  if (supports_decal > 1) {
    return texture(texture_sampler, texture_coords);
  }
  return IPSampleDecal(texture_sampler, texture_coords);
}

(Compare with advanced_blend.frag, porter_duff_blend.frag, and vertices_uber.glsl, which all check if (supports_decal > 0.0).)

Impact

Because supports_decal is at most 1.0, supports_decal > 1 always evaluates to false, causing framebuffer_blend.frag to unconditionally fall back to IPSampleDecal(texture_sampler, texture_coords) even when hardware SamplerAddressMode::kDecal is supported and configured on the sampler:

1. Unnecessary shader work: Every fragment executes the software [0, 1) UV bounds check in IPSampleDecal.
2. Unintended -0.475 mip bias: IPSampleDecal samples with kDefaultMipBias (-0.475). When drawing a downscaled mipmapped image (FilterQuality.medium/DlImageSampling::kMipmapLinear) with an advanced blend mode on a device that supports both framebuffer fetch and hardware decal addressing, the -0.475 LOD bias shifts mip selection toward a higher-resolution mip level than intended.

Expected Behavior

framebuffer_blend.frag should check if (supports_decal > 0.0) so that it samples texture(texture_sampler, texture_coords) directly when hardware decal addressing is supported.