font/freetype: introduce mutexes to ensure thread safety of Library and Face

For details see comments and FreeType docs @
https://freetype.org/freetype2/docs/reference/ft2-library_setup.html#ft_library
https://freetype.org/freetype2/docs/reference/ft2-face_creation.html#ft_face

tl;dr: FT_New_Face and FT_Done_Face require the Library to be locked for
thread safety, and FT_Load_Glyph and FT_Render_Glyph and friends need
the face to be locked for thread safety, since we're sharing faces
across threads.

src/font/SharedGridSet.zig

@@ -50,7 +50,7 @@ pub const InitError = Library.InitError;
 
 /// Initialize a new SharedGridSet.
 pub fn init(alloc: Allocator) InitError!SharedGridSet {
-    var font_lib = try Library.init();
+    var font_lib = try Library.init(alloc);
     errdefer font_lib.deinit();
 
     return .{