mach: initial support for high-level ECS applications

Adds experimental support for high-level ECS-based applications
following hexops/mach#349

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/engine.zig

@@ -0,0 +1,50 @@
+pub const Core = @import("Core.zig");
+pub const gpu = @import("gpu");
+pub const ecs = @import("ecs");
+
+/// The Mach engine ECS module. This enables access to `engine.get(.mach, .core)` `*Core` APIs, as
+/// to for example `.setOptions(.{.title = "foobar"})`, or to access the GPU device via
+/// `engine.get(.mach, .device)`
+pub const module = ecs.Module(.{
+    .globals = struct {
+        core: *Core,
+        device: gpu.Device,
+    },
+});
+
+pub fn App(
+    modules: anytype,
+    init: anytype, // fn (engine: *ecs.World(modules)) !void
+) type {
+    // TODO: validate modules.mach is the expected type.
+    // TODO: validate init has the right function signature
+
+    return struct {
+        engine: ecs.World(modules),
+
+        pub fn init(app: *@This(), core: *Core) !void {
+            app.* = .{
+                .engine = try ecs.World(modules).init(core.allocator),
+            };
+            app.*.engine.set(.mach, .core, core);
+            app.*.engine.set(.mach, .device, core.device);
+            try init(&app.engine);
+        }
+
+        pub fn deinit(app: *@This(), _: *Core) void {
+            app.engine.deinit();
+        }
+
+        pub fn update(app: *@This(), _: *Core) !void {
+            app.engine.tick();
+        }
+
+        pub fn resize(app: *@This(), core: *Core, width: u32, height: u32) !void {
+            _ = app;
+            _ = core;
+            _ = width;
+            _ = height;
+            // TODO: send resize messages to ECS modules
+        }
+    };
+}