Enable bitmap font usage under CoreText (#4115)

macOS bitmap-only fonts are a poorly documented format, which are often
distributed as `.dfont` or `.dfon` files. They use a 'bhed' table in
place of the usual 'head', but the table format is byte-identical, so
enabling the use of bitmap-only fonts only requires us to properly fetch
this table while calculating metrics.

ref: https://fontforge.org/docs/techref/bitmaponlysfnt.html

Reverts #3550 for obvious reasons, and may close issue #2168 because
this should now mean that bitmap fonts are properly supported under both
font backends due to #3837 - unless `otb` fonts are still not properly
supported under FreeType (they are not supported under CoreText because
CoreText does not know how to handle them).

I tested this change with the `.dfont` distribution of
[Cozette](https://github.com/slavfox/Cozette) v1.25.2 and saw no visual
issues.

src/font/discovery.zig

@@ -362,16 +362,9 @@ pub const CoreText = struct {
         const list = set.createMatchingFontDescriptors();
         defer list.release();
 
-        // Bring the list of descriptors in to zig land
-        var zig_list = try copyMatchingDescriptors(alloc, list);
-        errdefer alloc.free(zig_list);
-
-        // Filter them. We don't use `CTFontCollectionSetExclusionDescriptors`
-        // to do this because that requires a mutable collection. This way is
-        // much more straight forward.
-        zig_list = try alloc.realloc(zig_list, filterDescriptors(zig_list));
-
         // Sort our descriptors
+        const zig_list = try copyMatchingDescriptors(alloc, list);
+        errdefer alloc.free(zig_list);
         sortMatchingDescriptors(&desc, zig_list);
 
         return DiscoverIterator{
@@ -558,47 +551,13 @@ pub const CoreText = struct {
         for (0..result.len) |i| {
             result[i] = list.getValueAtIndex(macos.text.FontDescriptor, i);
 
-            // We need to retain because once the list
+            // We need to retain becauseonce the list is freed it will
-            // is freed it will release all its members.
+            // release all its members.
             result[i].retain();
         }
         return result;
     }
 
-    /// Filter any descriptors out of the list that aren't acceptable for
-    /// some reason or another (e.g. the font isn't in a format we can handle).
-    ///
-    /// Invalid descriptors are filled in from the end of
-    /// the list and the new length for the list is returned.
-    fn filterDescriptors(list: []*macos.text.FontDescriptor) usize {
-        var end = list.len;
-        var i: usize = 0;
-        while (i < end) {
-            if (validDescriptor(list[i])) {
-                i += 1;
-            } else {
-                list[i].release();
-                end -= 1;
-                list[i] = list[end];
-            }
-        }
-        return end;
-    }
-
-    /// Used by `filterDescriptors` to decide whether a descriptor is valid.
-    fn validDescriptor(desc: *macos.text.FontDescriptor) bool {
-        if (desc.copyAttribute(macos.text.FontAttribute.format)) |format| {
-            defer format.release();
-            var value: c_int = undefined;
-            assert(format.getValue(.int, &value));
-
-            // Bitmap fonts are not currently supported.
-            if (value == macos.text.c.kCTFontFormatBitmap) return false;
-        }
-
-        return true;
-    }
-
     fn sortMatchingDescriptors(
         desc: *const Descriptor,
         list: []*macos.text.FontDescriptor,

src/font/face/coretext.zig

@@ -515,8 +515,17 @@ pub const Face = struct {
     fn calcMetrics(ct_font: *macos.text.Font) CalcMetricsError!font.face.Metrics {
         // Read the 'head' table out of the font data.
         const head: opentype.Head = head: {
-            const tag = macos.text.FontTableTag.init("head");
+            // macOS bitmap-only fonts use a 'bhed' tag rather than 'head', but
-            const data = ct_font.copyTable(tag) orelse return error.CopyTableError;
+            // the table format is byte-identical to the 'head' table, so if we
+            // can't find 'head' we try 'bhed' instead before failing.
+            //
+            // ref: https://fontforge.org/docs/techref/bitmaponlysfnt.html
+            const head_tag = macos.text.FontTableTag.init("head");
+            const bhed_tag = macos.text.FontTableTag.init("bhed");
+            const data =
+                ct_font.copyTable(head_tag) orelse
+                ct_font.copyTable(bhed_tag) orelse
+                return error.CopyTableError;
             defer data.release();
             const ptr = data.getPointer();
             const len = data.getLength();