renderer: apply font-thicken to IME preedit text

IME preedit text was rendered with unthickened glyphs because
addPreeditCell omitted font_thicken options when calling
renderCodepoint, leaving them at their default false values.

Fixes #13758

src/renderer/generic.zig

@@ -3536,7 +3536,11 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                 @intCast(cp.codepoint),
                 .regular,
                 .text,
-                .{ .grid_metrics = self.grid_metrics },
+                .{
+                    .grid_metrics = self.grid_metrics,
+                    .thicken = self.config.font_thicken,
+                    .thicken_strength = self.config.font_thicken_strength,
+                },
             ) catch |err| {
                 log.warn("error rendering preedit glyph err={}", .{err});
                 return;