BlendFactor enum

One term of a blend equation.

The last four read a blend constant, and PassEncoder.setBlendColor is how it is set. blendColor, oneMinusBlendColor, blendAlpha and oneMinusBlendAlpha all multiply by that colour, which is transparent black until a pass says otherwise.

They used to be the one dead corner of these enums — no setter anywhere, and three backends disagreeing about what that meant: the software rasteriser threw, WebGL2 mapped them to CONSTANT_COLOR against a constant nobody had set, and Impeller handed them to flutter_gpu whose default is the same transparent black. Two of the three drew a plausible picture with a term missing from it, which is the failure this enum's own doc comment was written to warn about and could not prevent.

A backend may still not have the constant, and one here does not. Ask GraphicsDevice.supportsBlendColor before naming any of these four: flutter_gpu exposes no blend-constant setter at all, so the Impeller backend answers false and refuses a BlendState naming one rather than drawing the term as zero. That refusal is the promise the false answer carries, and the conformance check a blend constant reaches the blend, or is refused holds both halves of it.