coretext: exclude bitmap fonts from discovery

We do not currently support bitmap fonts in a real capacity, and they
often are missing some tables which we currently rely on for metrics,
and we don't handle the metrics calculations failing gracefully right
now.

src/font/discovery.zig

@@ -362,9 +362,16 @@ pub const CoreText = struct {
         const list = set.createMatchingFontDescriptors();
         defer list.release();
 
-        // Sort our descriptors
-        const zig_list = try copyMatchingDescriptors(alloc, list);
+        // 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
         sortMatchingDescriptors(&desc, zig_list);
 
         return DiscoverIterator{
@@ -551,13 +558,47 @@ pub const CoreText = struct {
         for (0..result.len) |i| {
             result[i] = list.getValueAtIndex(macos.text.FontDescriptor, i);
 
-            // We need to retain becauseonce the list is freed it will
-            // release all its members.
+            // We need to retain because once the list
+            // is freed it will 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,