font: Default to light hinting in FreeType (#9253)

This changes the default FreeType load target to `FT_LOAD_TARGET_LIGHT`,
giving the hinter a lighter touch in line with the default behavior in
most other GTK apps, and adds a load flag such that the old hinting
behavior can be restored via config.

As discussed in
https://github.com/ghostty-org/ghostty/issues/8674#issuecomment-3417082534.
However, this doesn't close that issue, as it still doesn't respect
custom Fontconfig settings. It's just a stopgap solution bringing
Ghostty's defaults more in line with the typical results of not
customizing Fontconfig.

src/config/Config.zig

@@ -476,6 +476,11 @@ pub const compatibility = std.StaticStringMap(
 ///
 ///   * `autohint` - Enable the freetype auto-hinter. Enabled by default.
 ///
+///   * `light` - Use a light hinting style, better preserving glyph shapes.
+///     This is the most common setting in GTK apps and therefore also Ghostty's
+///     default. This has no effect if `monochrome` is enabled. Enabled by
+///     default.
+///
 /// Example: `hinting`, `no-hinting`, `force-autohint`, `no-force-autohint`
 @"freetype-load-flags": FreetypeLoadFlags = .{},
 
@@ -7886,11 +7891,15 @@ pub const BackgroundImageFit = enum {
 pub const FreetypeLoadFlags = packed struct {
     // The defaults here at the time of writing this match the defaults
     // for Freetype itself. Ghostty hasn't made any opinionated changes
-    // to these defaults.
+    // to these defaults. (Strictly speaking, `light` isn't FreeType's
+    // own default, but appears to be the effective default with most
+    // Fontconfig-aware software using FreeType, so until Ghostty
+    // implements Fontconfig support we default to `light`.)
     hinting: bool = true,
     @"force-autohint": bool = false,
     monochrome: bool = false,
     autohint: bool = true,
+    light: bool = true,
 };
 
 /// See linux-cgroup

src/font/face/freetype.zig

@@ -378,6 +378,10 @@ pub const Face = struct {
             // else it won't look very good at all.
             .target_mono = self.load_flags.monochrome,
 
+            // Otherwise we select hinter based on the `light` flag.
+            .target_normal = !self.load_flags.light and !self.load_flags.monochrome,
+            .target_light = self.load_flags.light and !self.load_flags.monochrome,
+
             // NO_SVG set to true because we don't currently support rendering
             // SVG glyphs under FreeType, since that requires bundling another
             // dependency to handle rendering the SVG.
@@ -1143,7 +1147,7 @@ test {
             ft_font.glyphIndex('A').?,
             .{ .grid_metrics = font.Metrics.calc(ft_font.getMetrics()) },
         );
-        try testing.expectEqual(@as(u32, 20), g2.height);
+        try testing.expectEqual(@as(u32, 21), g2.height);
     }
 }