debugBilinearBackdropSampling top-level property

Forces the composition to bind the bilinear backdrop-reconstruction shader (final_render_bilinear_probe.frag) instead of the shipped nearest-neighbour default (final_render.frag).

ImageFilter.shader's backdrop sampler is nearest-neighbour by default (flutter#186945), so every displaced lookup snaps between texels. This flag exists to measure that against a hand-reconstructed bilinear alternative — see docs/reference/backdrop_sampling.md for the decision and the numbers behind it. The probe screen that flipped it lived in the workbench app, which has since been deleted.

Defaults to false. Nothing in production code ever sets it; it only matters to tooling that does. Setting it true before debugWarmUpBilinearBackdropSampling has completed throws — the shader it selects is not loaded eagerly (see GlassShaderId.core), precisely so no other consumer pays to load it.