Imgui (#20)

* vendor/cimgui

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

pkg/imgui/draw_data.zig

@@ -0,0 +1,20 @@
+const std = @import("std");
+const c = @import("c.zig");
+const imgui = @import("main.zig");
+const Allocator = std.mem.Allocator;
+
+pub const DrawData = opaque {
+    pub fn get() Allocator.Error!*DrawData {
+        return @ptrCast(
+            ?*DrawData,
+            c.igGetDrawData(),
+        ) orelse Allocator.Error.OutOfMemory;
+    }
+
+    pub inline fn cval(self: *DrawData) *c.ImGuiDrawData {
+        return @ptrCast(
+            *c.ImGuiDrawData,
+            @alignCast(@alignOf(c.ImGuiDrawData), self),
+        );
+    }
+};