Rationalize the flow of configuration updates.

[jcollie]

Right now when a configuration reload is triggered, the core App sends a change_config message to every apprt Surface.

ghostty/src/App.zig

Lines 145 to 148 in 3f715c2

	// Go through and update all of the surface configurations.
	for (self.surfaces.items) |surface| {
	try surface.core_surface.handleMessage(.{ .change_config = config });
	}

On GTK, Windows will have their updateConfig methods called multiple times (once for every surface contained in the window):

ghostty/src/apprt/gtk/App.zig

Lines 907 to 912 in 3f715c2

	.surface => |surface| surface: {
	const window = surface.rt_surface.container.window() orelse break :surface;
	window.updateConfig(new_config) catch |err| {
	log.warn("error updating config for window err={}", .{err});
	};
	},

This has not been a problem so far, but with trying to do more in the GTK apprt with the DerivedConfig model it could cause problems with extra allocs/deallocs, extra copying, and extra work by GTK as it processes the repeated changes.

The flow of configuration reloads should be rationalized so that duplicate work is not being done.