Imgui (#20)

* vendor/cimgui

* Add a "dev mode" window which for now is just imgui demo

src/DevMode.zig

@@ -0,0 +1,38 @@
+//! This file implements the "dev mode" interface for the terminal. This
+//! includes state managements and rendering.
+const DevMode = @This();
+
+const imgui = @import("imgui");
+
+/// If this is false, the rest of the terminal will be compiled without
+/// dev mode support at all.
+pub const enabled = true;
+
+/// The global DevMode instance that can be used app-wide. Assume all functions
+/// are NOT thread-safe unless otherwise noted.
+pub var instance: DevMode = .{};
+
+/// Whether to show the dev mode UI currently.
+visible: bool = false,
+
+/// Update the state associated with the dev mode. This should generally
+/// only be called paired with a render since it otherwise wastes CPU
+/// cycles.
+pub fn update(self: DevMode) void {
+    _ = self;
+    imgui.ImplOpenGL3.newFrame();
+    imgui.ImplGlfw.newFrame();
+    imgui.newFrame();
+
+    // Just demo for now
+    imgui.showDemoWindow(null);
+}
+
+/// Render the scene and return the draw data. The caller must be imgui-aware
+/// in order to render the draw data. This lets this file be renderer/backend
+/// agnostic.
+pub fn render(self: DevMode) !*imgui.DrawData {
+    _ = self;
+    imgui.render();
+    return try imgui.DrawData.get();
+}