EPS and .ai import: images with TIFF predictor 2 draw their samples at every bit depth

The PostScript interpreter that reads EPS and PostScript .ai files
accepts a FlateDecode or LZWDecode filter with Predictor 2 at
BitsPerComponent 1, 2, 4, 8 and 16, but undid the horizontal
differencing only at 8 bits. At the other depths the filter gave back
the stored differences, and the image drew them instead of its
samples.

The predictor is now undone at every depth it accepts, as ISO 32000-1
§7.4.4.4 describes: each component adds the same component of the
pixel to its left, within its bit width, and each row starts again.
16-bit components add as big-endian 16-bit numbers. The padding bits
at the end of a row stay as they are, and 8-bit data takes the same
path as before. The PNG predictors are unchanged.

Docs: docs/target-app-parity.md.

crates/engine/src/cmd/fileio/tests_epsimport.rs

@@ -1,6 +1,8 @@
 //! EPS and PostScript files opened and placed: our own EPS files restore the document they carry,
 //! others are read by the PostScript interpreter or come in as their preview.
 
+use std::io::Write as _;
+
 use serde_json::{Value, json};
 
 use super::tests_svgedit::{comparable, rich};
@@ -93,6 +95,33 @@ fn cmyk_images_keep_their_inks_when_an_eps_opens_or_is_placed() {
     assert_eq!(images(&s), [want.clone(), want]);
 }
 
+/// A 4-bit gray image compressed with Flate and TIFF predictor 2 draws the samples it had before
+/// differencing, in an EPS and in a PostScript `.ai`.
+#[test]
+fn tiff_predicted_4_bit_images_open_from_eps_and_postscript_ai_files() {
+    // Two rows of four samples, 0 5 15 3 and 9 9 2 0, differenced to 0 5 10 4 and 9 0 9 14.
+    let mut z = flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::default());
+    z.write_all(&[0x05, 0xa4, 0x90, 0x9e]).unwrap();
+    let hex: String = z.finish().unwrap().iter().map(|b| format!("{b:02x}")).collect();
+    let ps = format!(
+        "%!PS-Adobe-3.0 EPSF-3.0\n%%BoundingBox: 0 0 40 20\n%%EndComments\n40 20 scale 4 2 4 [4 0 0 -2 0 2] currentfile /ASCIIHexDecode filter \
+         << /Predictor 2 /Colors 1 /BitsPerComponent 4 /Columns 4 >> /FlateDecode filter image\n{hex}>\nshowpage\n%%EOF\n"
+    );
+    for name in ["predicted.eps", "predicted.ai"] {
+        let mut s = Session::new();
+        let r = open(&mut s, name, ps.as_bytes()).unwrap();
+        assert_eq!(r["warnings"], json!([]), "{name}: {r}");
+        let d = &s.doc().unwrap().doc;
+        let mut grays = vec![];
+        d.walk(|n| {
+            if let vectorcraft_doc::NodeKind::Image(im) = &n.kind {
+                grays = image::load_from_memory(&d.images[&im.key].bytes).unwrap().to_luma8().into_raw();
+            }
+        });
+        assert_eq!(grays, [0, 5, 15, 3, 9, 9, 2, 0].map(|v| v * 17), "{name}");
+    }
+}
+
 #[test]
 fn an_unreadable_eps_file_comes_in_as_its_preview() {
     let mut s = Session::new();