move screen resize to a renderer mailbox message

src/renderer/message.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const font = @import("../font/main.zig");
+const renderer = @import("../renderer.zig");
 
 /// The messages that can be sent to a renderer thread.
 pub const Message = union(enum) {
@@ -18,4 +19,7 @@ pub const Message = union(enum) {
     /// send a grid size change message back to the window thread if
     /// the size changes.
     font_size: font.face.DesiredSize,
+
+    /// Change the screen size.
+    screen_size: renderer.ScreenSize,
 };