createTexture method
A brand-new texture matching spec, with the single TextureHandle that
will ever stand for it.
Implementation
@override
TextureHandle createTexture(RenderTargetSpec spec) {
createdTextures.add(spec);
return TextureHandle(
backend: 'fake ${_serial++}',
width: spec.width,
height: spec.height,
format: spec.format,
sampleCount: spec.sampleCount,
storageMode: spec.storageMode,
);
}