supportsBlendColor property
Whether PassEncoder.setBlendColor reaches the hardware.
The four BlendFactor values that read a blend constant are only usable
where this is true. It is not a property of the hardware — Metal, Vulkan
and GL all have the constant — but of what the backend can reach: the
Impeller backend goes through flutter_gpu, whose RenderPass exposes no
blend-constant setter at all, so it answers false and there is nothing it
can do about that from Dart.
False carries a promise, and it is a refusal. A backend that answers
false throws an UnsupportedError from PassEncoder.setBlendColor, and
throws from PassEncoder.setBlend when the state names one of the four —
see BlendState.usesBlendColor. Drawing the term as zero instead is the
exact failure this capability was added to end: a plausible picture with a
term missing from it, and nothing anywhere saying so.
Implementation
@override
// Recorded, not evaluated: this device blends nothing, so the honest answer
// is the one that lets a caller under test set the constant and be recorded
// doing it.
bool get supportsBlendColor => true;