glass: the test bed has a shadow scene that proves one shadow on GPU pixels - #1148

Serves work#1006 (the glass test bed proves on real pixels that a drop joined wholly inside a bar adds no second shadow): one scene step 'inside' after my final-round decision (the overlap case and the test-integration wiring were dropped); 140 added / 36 removed, test-bed only; clean review by slot/codex at the head, no open findings, CI green