add: Direct Renderer implementation rendering the fixed size of the root Container

A corresponding example has been added, which is very minimalistic as of now,
but will add further functionality to test the corresponding workflow that is
usual in `zterm` such that it in the best case only a swap in the renderer to
switch from alternate mode drawing to direct drawing.

examples/direct.zig

@@ -0,0 +1,46 @@
+pub fn main() !void {
+    errdefer |err| log.err("Application Error: {any}", .{err});
+
+    var allocator: std.heap.DebugAllocator(.{}) = .init;
+    defer if (allocator.deinit() == .leak) log.err("memory leak", .{});
+
+    const gpa = allocator.allocator();
+
+    var app: App = .init(.{}, .{});
+    var renderer = zterm.Renderer.Direct.init(gpa);
+    defer renderer.deinit();
+
+    var container: App.Container = try .init(gpa, .{
+        .layout = .{
+            .direction = .horizontal,
+            .separator = .{ .enabled = true },
+        },
+        .size = .{
+            .dim = .{ .y = 30 },
+            .grow = .horizontal_only,
+        },
+    }, .{});
+    defer container.deinit();
+
+    try container.append(try .init(gpa, .{
+        .rectangle = .{ .fill = .blue },
+    }, .{}));
+
+    try container.append(try .init(gpa, .{
+        .rectangle = .{ .fill = .green },
+    }, .{}));
+
+    container.resize(&app.model, try renderer.resize());
+    container.reposition(&app.model, .{});
+    try renderer.render(@TypeOf(container), &container, App.Model, &app.model);
+    try renderer.flush();
+}
+
+pub const panic = App.panic_handler;
+const log = std.log.scoped(.default);
+
+const std = @import("std");
+const assert = std.debug.assert;
+const zterm = @import("zterm");
+const input = zterm.input;
+const App = zterm.App(struct {}, union(enum) {});