Core: add .present_frame event replacing swapbuffers call

Note that on e.g. web platform, swapbuffers is not an explicit call.
We also need a signal that the frame has been submitted and finished,
and this is it.

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

examples/custom-renderer/Game.zig

@@ -47,10 +47,6 @@ fn init(
     renderer: *Renderer.Mod,
     game: *Mod,
 ) !void {
-    // The Mach .core is where we set window options, etc.
-    // TODO(important): replace this API with something better
-    mach.core.setTitle("Hello, ECS!");
-
     // Create our player entity.
     const player = try core.newEntity();