Per-element opacity sanitizer. Mirrors GLSL sanitizeAlpha: NaN/Inf
route to the 1.0 opaque identity (corruption stays LOUD), finite
values clamp to [0, 1] — alpha is opacity, never HDR (Python pins the
range at write; this guards hand-crafted zarr). The clamp keeps the
zero boundary CONTINUOUS (a -1e-4 epsilon vanishes like +0.0 renders,
instead of jumping to full opacity) and keeps the value
mediump-varying-safe.
Per-element opacity sanitizer. Mirrors GLSL
sanitizeAlpha: NaN/Inf route to the 1.0 opaque identity (corruption stays LOUD), finite values clamp to [0, 1] — alpha is opacity, never HDR (Python pins the range at write; this guards hand-crafted zarr). The clamp keeps the zero boundary CONTINUOUS (a -1e-4 epsilon vanishes like +0.0 renders, instead of jumping to full opacity) and keeps the value mediump-varying-safe.