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

src/exif.ts

@@ -1,7 +1,7 @@
 // EXIF in an original's bytes, read with exifreader, which reads it from JPEG,
 // HEIC/HEIF, AVIF, PNG, WebP and the other image formats it supports.
-// `backup-metadata --exif` records every EXIF tag it finds; `Photo.exif()`
-// returns the common fields picked from them here.
+// `backup-metadata --exif` records every EXIF tag it finds except the
+// thumbnail's; `Photo.exif()` returns the common fields picked from them here.
 
 import ExifReader, { type ExpandedTags } from "exifreader";
 
@@ -9,7 +9,11 @@ import ExifReader, { type ExpandedTags } from "exifreader";
 // them (`gps`), and where the EXIF block lies in `bytes` (`metadataRange`).
 // Undefined when exifreader cannot read the file at all, such as a video. An
 // EXIF block it finds but reads no tag from comes back as an empty `exif`.
-// Each tag's `computed` holds its value as a plain string or number.
+// `exif` holds every tag except the thumbnail's; a tag exifreader has no name
+// for is keyed `undefined-<tag number>`. Each tag's `computed` holds its value
+// as a string or number, or as an array of them for a tag with several values,
+// such as `GPSLatitude`'s `[40, 26, 46]`. A fraction with a zero denominator
+// computes to null.
 export const readExifTags = (bytes: Uint8Array): ExpandedTags | undefined => {
     try {
         return ExifReader.loadView(
@@ -18,6 +22,7 @@ export const readExifTags = (bytes: Uint8Array): ExpandedTags | undefined => {
                 expanded: true,
                 computed: true,
                 includeOffsets: true,
+                includeUnknown: true,
                 includeTags: { exif: true, gps: true },
             },
         );
@@ -77,6 +82,7 @@ export const readPhotoExif = (bytes: Uint8Array): PhotoExif => {
     const tags = readExifTags(bytes);
     const exif = tags?.exif;
     const gps = tags?.gps;
+    const altitude = asNumber(exif?.GPSAltitude?.computed);
     const fields: PhotoExif = {
         make: asString(exif?.Make?.computed),
         model: asString(exif?.Model?.computed),
@@ -91,7 +97,11 @@ export const readPhotoExif = (bytes: Uint8Array): PhotoExif => {
         orientation: asNumber(exif?.Orientation?.computed),
         gpsLatitude: asNumber(gps?.Latitude),
         gpsLongitude: asNumber(gps?.Longitude),
-        gpsAltitude: asNumber(gps?.Altitude),
+        // A GPSAltitudeRef of 1 means the altitude is below sea level.
+        gpsAltitude:
+            altitude !== undefined && exif?.GPSAltitudeRef?.value === 1
+                ? -altitude
+                : altitude,
     };
     // Leave out what the file lacks, so a missing field is absent rather than
     // present and undefined.