Add fuzzer for the encoder

fuzz/.gitignore

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+target
+corpus
+artifacts

fuzz/Cargo.toml

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+[package]
+name = "qoi-fast-fuzz"
+version = "0.1.0"
+authors = ["Ivan Smirnov <rust@ivan.smirnov.ie>"]
+publish = false
+edition = "2021"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+# internal
+qoi-fast = { path = ".." }
+# external
+libfuzzer-sys = "0.4"
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "qoi_encode"
+path = "fuzz_targets/qoi_encode.rs"
+test = false
+doc = false

fuzz/fuzz_targets/qoi_encode.rs

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+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|input: (bool, u8, &[u8])| {
+    let (is_4, w_frac, data) = input;
+    let channels = if is_4 { 4 } else { 3 };
+    let size = data.len();
+    let n_pixels = size / channels as usize;
+    let (w, h) = if n_pixels == 0 {
+        (0, 0)
+    } else {
+        let w = ((n_pixels * (1 + w_frac as usize)) / 256).max(1);
+        let h = n_pixels / w;
+        (w, h)
+    };
+    let out = qoi_fast::qoi_encode_to_vec(
+        &data[..(w * h * channels as usize)],
+        w as u32,
+        h as u32,
+        channels,
+        0,
+    );
+    if w * h != 0 {
+        let out = out.unwrap();
+        assert!(out.len() <= qoi_fast::encode_size_required(w as u32, h as u32, channels));
+    } else {
+        assert!(out.is_err());
+    }
+});