mach: correct vsync mode setting

This correctly sets presentation modes for vsync, both at startup and at runtime via
a `setOptions` request.

Note: There may still be platforms where setting vsync is not enough, and a frame rate
limiter is needed to achieve proper synchronization. This is tracked in hexops/mach#444
and not fixed by this change.

Fixes hexops/mach#307

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

src/platform/wasm.zig

@@ -286,7 +286,7 @@ export fn wasmInit() void {
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
     const allocator = gpa.allocator();
 
-    core = Core.init(allocator) catch unreachable;
+    Core.init(allocator, &core) catch unreachable;
     app.init(&core) catch {};
 }