font: fontconfig always prefer monospace in pattern

Fixes #1984

pkg/fontconfig/common.zig

@@ -23,6 +23,13 @@ pub const Slant = enum(c_uint) {
     oblique = c.FC_SLANT_OBLIQUE,
 };
 
+pub const Spacing = enum(c_uint) {
+    proportional = c.FC_PROPORTIONAL,
+    dual = c.FC_DUAL,
+    mono = c.FC_MONO,
+    charcell = c.FC_CHARCELL,
+};
+
 pub const Property = enum {
     family,
     style,

src/font/discovery.zig

@@ -139,6 +139,15 @@ pub const Descriptor = struct {
             false,
         ));
 
+        // For fontconfig, we always add monospace in the pattern. Since
+        // fontconfig sorts by closeness to the pattern, this doesn't fully
+        // exclude non-monospace but helps prefer it.
+        assert(pat.add(
+            .spacing,
+            .{ .integer = @intFromEnum(fontconfig.Spacing.mono) },
+            false,
+        ));
+
         return pat;
     }
 
@@ -277,7 +286,7 @@ pub const Fontconfig = struct {
         if (res.result != .match) return error.FontConfigFailed;
         errdefer res.fs.destroy();
 
-        return DiscoverIterator{
+        return .{
             .config = self.fc_config,
             .pattern = pat,
             .set = res.fs,