Add custom layout creation.

src/Terminal.zig

@@ -54,7 +54,14 @@ on_focus: *const fn (ctx: ?*anyopaque) void,
 
 ctx: ?*anyopaque = null,
 
-pub fn create(alloc: std.mem.Allocator, cbs: Pane.Callbacks) !*Terminal {
+/// What a layout can specify for a terminal pane.
+pub const Options = Session.Options;
+
+pub fn create(
+    alloc: std.mem.Allocator,
+    opts: Options,
+    cbs: Pane.Callbacks,
+) !*Terminal {
     const self = try alloc.create(Terminal);
     errdefer alloc.destroy(self);
 
@@ -76,7 +83,7 @@ pub fn create(alloc: std.mem.Allocator, cbs: Pane.Callbacks) !*Terminal {
 
     // The grid size follows the widget size, but we need a starting point
     // for the session before the widget has ever been allocated.
-    self.session = try .create(alloc, 80, 24, .{
+    self.session = try .create(alloc, 80, 24, opts, .{
         .on_damage = &onDamage,
         .on_title = &onSessionTitle,
         .on_exit = &onSessionExit,