bench: add decode benchmark for extended bit depths

Measures decode throughput for 1024x1024 gray images at various bit
depths (3-24 bit). Requires pre-generated images in /tmp/bench-tiff/
(see stderr instructions for generation).

Cargo.toml

@@ -54,6 +54,10 @@ harness = false
 name = "encode"
 harness = false
 
+[[bench]]
+name = "decode"
+harness = false
+
 [[test]]
 name = "decode_libtiffpic"
 path = "tests/decode_libtiffpic.rs"