Read GPSAltitude without its reference tag; keep unnamed EXIF tags

exif() takes the altitude from the GPSAltitude tag and negates it only when
GPSAltitudeRef is 1, so a file with GPSAltitude and no GPSAltitudeRef keeps its
altitude, above sea level. A test covers that file.

backup-metadata --exif keeps the EXIF tags exifreader has no name for. The
comments in src/exif.ts say the thumbnail's tags are not kept and that a tag
with several values computes to an array.

Model: opus-5-5

test/cli/metadata-exif.test.ts

@@ -46,6 +46,23 @@ const TIFF_UNSET_DATE = [
     ...new TextEncoder().encode("0000:00:00 00:00:00\0"), // at 56
 ];
 
+// A big-endian TIFF block holding a GPSAltitude of 12.5 m and no
+// GPSAltitudeRef.
+const TIFF_ALTITUDE_WITHOUT_REF = [
+    ...[0x4d, 0x4d, 0x00, 0x2a, 0x00, 0x00, 0x00, 0x08], // the first IFD at 8
+    // The first IFD, at 8: one entry, then no next IFD.
+    ...[0x00, 0x01],
+    // The GPS IFD's offset (0x8825), LONG, 26.
+    ...[0x88, 0x25, 0x00, 0x04, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x1a],
+    ...[0x00, 0x00, 0x00, 0x00],
+    // The GPS IFD, at 26: one entry, then no next IFD.
+    ...[0x00, 0x01],
+    // GPSAltitude (0x0006), one RATIONAL at 44.
+    ...[0x00, 0x06, 0x00, 0x05, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x2c],
+    ...[0x00, 0x00, 0x00, 0x00],
+    ...[0x00, 0x00, 0x00, 0x19, 0x00, 0x00, 0x00, 0x02], // 25/2, at 44
+];
+
 // An APP1 segment whose length field matches its data.
 const app1 = (data: number[]): number[] => {
     const len = data.length + 2;
@@ -70,6 +87,13 @@ describe("readPhotoExif", () => {
         });
     });
 
+    it("reads a GPSAltitude without GPSAltitudeRef as above sea level", () => {
+        const data = [...EXIF_HEADER, ...TIFF_ALTITUDE_WITHOUT_REF];
+        expect(readPhotoExif(bytes(SOI, app1(data), SOS))).toStrictEqual({
+            gpsAltitude: 12.5,
+        });
+    });
+
     it.each([
         [
             "a file that is not an image",