fix: demote off_l16.tif to smoke test (LogLuv exp() platform variance)

log16_to_y() uses f32::exp() which is not required to be correctly
rounded by IEEE 754. Apple libm on arm64 differs by 1 ULP from glibc
on x86_64, producing different CRC32 checksums for the decoded output.

Same issue as off_luv32.tif which was already demoted in commit 617dc6e.

A deterministic LUT-based approach could fix both in a follow-up.

tests/decode_libtiffpic.rs

@@ -184,7 +184,7 @@ const FILES: &[(&str, Option<u32>)] = &[
     ("tests/libtiffpic/jim___dg.tif", Some(0x8c6a5e8f)),
     ("tests/libtiffpic/jim___gg.tif", Some(0x8c6a5e8f)),
     ("tests/libtiffpic/ladoga.tif", Some(0x9b118511)),
-    ("tests/libtiffpic/off_l16.tif", Some(0xd434a2d9)),
+    ("tests/libtiffpic/off_l16.tif", Some(0)), // LogLuv decoder output varies across platforms
     ("tests/libtiffpic/off_luv24.tif", Some(0x2bee71a2)),
     ("tests/libtiffpic/off_luv32.tif", Some(0)), // LogLuv decoder output varies across platforms
     ("tests/libtiffpic/oxford.tif", Some(0x355d2e3e)),