Core: fix potential printTitle leak

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/Core.zig

@@ -66,6 +66,11 @@ pub fn printTitle(
     comptime fmt: []const u8,
     args: anytype,
 ) !void {
+    // Free any previous window title slice
+    // TODO: reuse allocations
+    if (core.get(window_id, .title)) |slice| core.state().allocator.free(slice);
+
+    // Allocate and assign a new window title slice.
     const slice = try std.fmt.allocPrintZ(core.state().allocator, fmt, args);
     try core.set(window_id, .title, slice);
 }