Core: use an explicit .start event sent by app to begin .tick events

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

examples/core-custom-entrypoint/App.zig

@@ -58,6 +58,8 @@ fn init(game: *Mod, core: *mach.Core.Mod) !void {
         .pipeline = pipeline,
     });
     try updateWindowTitle(core);
+
+    core.send(.start, .{});
 }
 
 // TODO(important): remove need for returning an error here

examples/custom-renderer/App.zig

@@ -74,6 +74,8 @@ fn init(
         .spawn_timer = try mach.Timer.start(),
         .player = player,
     });
+
+    core.send(.start, .{});
 }
 
 // TODO(important): remove need for returning an error here

examples/glyphs/App.zig

@@ -46,7 +46,7 @@ fn deinit(core: *mach.Core.Mod, sprite_pipeline: *gfx.SpritePipeline.Mod, glyphs
     core.send(.deinit, .{});
 }
 
-fn init(sprite_pipeline: *gfx.SpritePipeline.Mod, glyphs: *Glyphs.Mod, game: *Mod) !void {
+fn init(core: *mach.Core.Mod, sprite_pipeline: *gfx.SpritePipeline.Mod, glyphs: *Glyphs.Mod, game: *Mod) !void {
     sprite_pipeline.send(.init, .{});
     glyphs.send(.init, .{});
 
@@ -55,6 +55,8 @@ fn init(sprite_pipeline: *gfx.SpritePipeline.Mod, glyphs: *Glyphs.Mod, game: *Mo
 
     // Run our init code after glyphs module is initialized.
     game.send(.after_init, .{});
+
+    core.send(.start, .{});
 }
 
 fn afterInit(

examples/piano/App.zig

@@ -36,7 +36,7 @@ pub const components = .{
 
 ghost_key_mode: bool = false,
 
-fn init(audio: *mach.Audio.Mod, app: *Mod) void {
+fn init(core: *mach.Core.Mod, audio: *mach.Audio.Mod, app: *Mod) void {
     // Initialize audio module, telling it to send our module's .audio_state_change event when an
     // entity's sound stops playing
     audio.send(.init, .{app.event(.audio_state_change)});
@@ -49,6 +49,8 @@ fn init(audio: *mach.Audio.Mod, app: *Mod) void {
     std.debug.print("[spacebar]   enable ghost-key mode (demonstrate seamless back-to-back sound playback)\n", .{});
     std.debug.print("[arrow up]   increase volume 10%\n", .{});
     std.debug.print("[arrow down] decrease volume 10%\n", .{});
+
+    core.send(.start, .{});
 }
 
 fn deinit(core: *mach.Core.Mod, audio: *mach.Audio.Mod) void {

examples/sprite/App.zig

@@ -89,6 +89,8 @@ fn init(
         .allocator = allocator,
         .pipeline = pipeline,
     });
+
+    core.send(.start, .{});
 }
 
 fn tick(

examples/text/App.zig

@@ -127,6 +127,8 @@ fn init(
         .allocator = allocator,
         .pipeline = pipeline,
     });
+
+    core.send(.start, .{});
 }
 
 fn tick(