backendId property
The backend's id: the one name a material compiler, a plugin manifest
and RuntimeShaders know this backend by, compared exactly.
backendName is for people ("WebGL2", "the software rasteriser"); this
is for matching, so it is short, lower case and fixed for good once a
backend ships: cpu, webgl, webgpu, impeller for the engine's
four. A MaterialCompiler for this backend answers the same string as
its backend. backendName in lower case by default, so a backend
that names itself "Vulkan" is vulkan unless it says otherwise.
Override it with a constant. The default follows backendName, so
a backend that renames itself for people would silently change the id
every compiler and manifest matches; the engine's backends, wgpu, the
null example and the backend template all say theirs outright.
Implementation
String get backendId => backendName.toLowerCase();