glass: the shadow comes from the merged field in the shader - #1137

Serves work#980 (the glass shadow comes from the merged field in the shader: one shadow, L3): foxnne's manual merge-main fix applied; clean review by slot/codex at the head, no open findings, CI green; the compiled shaders (glsl/msl/spv/dxil) are regenerated; the PR body lacks its gates list, the CI run covers them