window caches all sizing so it doesn't depend on renderer state

src/renderer/Options.zig

@@ -2,6 +2,7 @@
 
 const font = @import("../font/main.zig");
 const renderer = @import("../renderer.zig");
+const Window = @import("../Window.zig");
 
 /// The font group that should be used.
 font_group: *font.GroupCache,
@@ -9,6 +10,10 @@ font_group: *font.GroupCache,
 /// Padding options for the viewport.
 padding: Padding,
 
+/// The mailbox for sending the window messages. This is only valid
+/// once the thread has started and should not be used outside of the thread.
+window_mailbox: Window.Mailbox,
+
 pub const Padding = struct {
     // Explicit padding options, in pixels. The windowing thread is
     // expected to convert points to pixels for a given DPI.