Test reading planes of a whole image

tests/decode_planes.rs

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+use tiff::decoder::Decoder;
+
+use std::fs::File;
+use std::path::PathBuf;
+
+const TEST_IMAGE_DIR: &str = "./tests/images/";
+
+#[test]
+fn read_all_planes() {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("planar-rgb-u8.tif");
+    let file = File::open(path).unwrap();
+    let io = std::io::BufReader::new(file);
+
+    let mut tif = Decoder::new(io).unwrap();
+    let layout = tif.image_buffer_layout().unwrap();
+    assert_eq!(layout.planes, 3);
+    let plane_stride = layout.plane_stride.map_or(0, |x| x.get());
+
+    let mut buffer = vec![0u8; layout.planes * plane_stride];
+    tif.read_image_bytes(&mut buffer).unwrap();
+
+    // Mainly you can see: these are different and non-zero. Otherwise these magic constants just
+    // depend on the file itself that is being used.
+    const PLANE_EXCPECTED_SUMS: [u32; 3] = [15417630, 13007788, 11103530];
+
+    for (plane, expected) in (0..layout.planes).zip(PLANE_EXCPECTED_SUMS) {
+        let plane_data = &buffer[plane_stride * plane..][..layout.len];
+        let sum: u32 = plane_data.iter().copied().map(u32::from).sum();
+        assert_eq!(sum, expected);
+    }
+}